Preface: I'm no talented runner or anything, and I've never competed for wins in big races, so my input probably doesn't mean much.
I think, except in somewhat rare cases, that even if one is capable of winning a race, that it is stupid to actually sprint for more than a few seconds at the beginning of a 5k-10k XC race. I acknowledge that sometimes it will be necessary/appropriate to sprint for more than a few seconds (but still definitely not 1/4 mile as a previous poster said), in order to gain decent position for a narrowing course, or something like that. But otherwise, I may be totally making this up, but haven't there been studies done which say that the ideal "sprint" at the beginning of a race shouldn't be more than 10 seconds or so, maybe 15 at the most, before it will negatively affect your ability to hold your race pace later in the race? Or something like that? I don't know the physics or specifics of this, but it makes sense to me.
Obviously, it is necessary to remain in contact with the front of the race if you are going for the win, but if they are sprinting out way ahead in the beginning, it is extremely likely they will be negatively affected by this later in the race and you don't have to worry about it. I think that as long as you can get decent positioning in the first 10-20 seconds of the race, in MOST cases, any potential race winner is going to be fine, and indeed, better off, by relaxing into race pace sooner rather than later.
For those other cases, you just have to be smart. But in my opinion it is more often than not dumb to have a sprint-out at the beginning of a 5k-10k race, not smart.