I've never used a filter for paintball photos. This is what my front element looks like:
This lens has been hit at least 15-20 times head on.
As you can see, it's a little beat but there are no major scratches. The smudges still need to be cleaned a bit to prevent unnatural flaring but I have lens tissues for that. I'm not a pixel peeper, but over the years I've noticed that unless my front element is really dirty I don't lose any contrast or iq.
On the flip side, I love my f4 and telephoto work isn't what I do for the most part, so I plan on keeping this lens for a while. That being said, an exposed front element is bound to affect your resale.
Here's what I'd say as far as getting good shots goes:
1) Shoot on the same level as your subject.
looks much less intimate than this shot:
2) Focus is tricky. Be sure to use servo focusing or whatever your camera's mode for focusing as the subject moves is.
3) If you can get your hands on a camera with good burst and a quick AF system (the 1d series, for example), do it. Otherwise just time your shots well.
4) Shutter speed>1/500ish most of the time. 1/2000 will freeze the ball pretty well.
5) Don't be afraid of higher iso if you need it. I've shot as high as 800 in good light to accommodate for other settings.
5) Get your hands on good software for post. Lightroom, photoshop, aperture, etc.
6) Paintball photography is a bit limiting in some respects, so let your mind go to work when you're shooting. Capturing emotion and interesting subject matter takes a certain kind of skill, and many photos lack that if you're shooting without a thought process.
7) If you're spending so much on equipment, pick yourself up a clear lens for your mask. It makes exposure adjustment so much easier.