Ever ask yourself whether you should buy into the UV filter for protection spiel? Wondering if a hood might be better? Here's my personal ideology on which to use when.
When to use a hood
Short answer: Always.
Long answer: Well, you don't technically always need a hood on your lens, but it might be a good idea. Not only does the hood prevent light beams from causing flare and contrast degradation (depending on the lens, of course), but they also protect your precious front element from bumps and dings off of that one corner that you could swear jumps out at you, or that tense moment after your camera was in your hands and now it's on the floor.
Hoods can help in both situations, absorbing the blow more more effectively, provided the impact would have been on the front element had you not had a hood on. This is because they're typically made of plastic or rubber, and the amount of flex when an impact is taken by the hood distributes the force of the impact over a (relatively) much longer time span than if it was just the glass or metal.
All of this is assuming that your hood extends a good bit forward from your lens. If your lens is a wide angle, then chances are the hood will add only a little protection, but it's better than nothing!
When not to use a hood
I can't think of a reason not to use a hood other than you don't have one for that lens. In which case, go buy one! However, some may feel as though a hood makes their camera more conspicuous, especially in the case of hoods for telephoto lenses. Specifically in the case of portrait photography, a subject can become more intimidated by a lens with a hood than one without. I guess it's personal choice, and my personal choice is for the pros over the cons.
When to use a UV filter
Stores love to claim that a UV filter will protect your lens, if something should happen to it. Read that last part of the sentence as though it's being spoken by a large, mob-boss-type guy. That's how I feel about them.
Imagine this scenario:
You're shooting a kids soccer game, and suddenly, a soccer ball is hurling itself right towards you and your camera. You're unable to get out of the way.
With a UV filter on, chances are it's going to break. Nobody disagrees about this part, as that's the point. The filter breaks, not your lens. No harm, no foul. Except, physics says that... Well, you know how eggs are weak on their sides, right? That's why we crack them there. However, you don't see anyone cracking eggs on their curved ends. Why is that? That's because a convex curve (one going opposite the direction of the oncoming object) is stronger than a flat one. Look at your lens. Chances are that the front element is curved. Filters are flat. When the filter breaks, it doesn't mean your front element is going to break.
Furthermore, if your filter breaks, you could be experiencing some filter-induced scratches on your front element. When the ball breaks the filter, the sharp edges are still moving towards your lens. How do I know? The filter broke, which means it couldn't dissipate all of the force from the ball, which means there's still force moving towards your front element. Those sharp edges hit your front element and could cause scratches, which is ironic, because that's what it's supposed to prevent, right?
I've illustrated this with my amazing MS Paint skills...
What if you had a hood on, instead of a filter? Well, that'd be a different scenario. Since a soccer ball is probably larger than your hood, the moment of impact might break the plastic holding the hood on, but cause no damage to your lens. If the lens is being hit by something like a baseball, your curved front element should withstand more force than a flat filter, but if your lens is being hit by a baseball with a hood on, then I'm pretty sure you couldn't do anything to stop it. Fate has decided.
So when should you really use a filter?
You're expecting me to say never, right?
Wrong. If you're going to be in a dusty, rainy, mucky, oily, basically any kind of environment where there may be a spray that could get on your front element, then you should use a UV filter. Or, if conditions permit, make the protection do double duty and use a polarizer or a neutral density filter. Still use a hood, of course, because it's the real protection, but it's a lot more comforting to know that you're cleaning off a filter instead of your front element. Not to say that it wouldn't withstand it, but that's an easy way to get scratches.
We've seen time and time again, DigitalRev TV proving that even entry level DSLR's are rugged. That doesn't mean we want to intentionally test this, so why use a filter when you don't really need to? If you want protection from bumps and scrapes, get a hood, put it on, and voila. If you want to protect against foreign substances getting stuck on your lens, grab a filter that suits your needs. If you don't have a hood, but want protection, be careful, don't use a UV filter just for "in case of impact" as it might end up causing more harm than good if there is such an impact.
Note: I'm not saying filters are bad. They do have jobs. UV filters in the digital photography realm, however, don't do a job, unless it's protecting against gunk on your front element when you don't need a neutral density or circular polarizer. Shops will convince you to buy a UV filter in case of drops or bumps, but don't buy into it. Also, you should be treating your camera gear well enough that you don't need any of this, but it's a nice insurance policy.