In fact, if you install a softmod you also void your warranty since the user license explicitly states you are only allowed to run xbox signed software. Use of any unsigned software voids your warranty.
If however you can still cleanup your softmod when your xbox has some sort of malfunction (like the dvd drive won't read discs anymore fe) you could get away with it without the factory guys knowing about it...
But if you are scared about losing your warranty, don't mod at all.
I think however the risc of fucking up completely is very low, especially if you use the gamesave hacks for exploiting your xbox (iso hotswapping). Combined with the fact that an xbox does not cost *that* much anymore, i see no reason in why one wouldn't mod it