Of course density varies with temp but that doesn't mean it really matters. When you have graduations, whether etched into the sidewall, stickers on a level gauge or lines on a story board, you aren't accounting for a change based on temperature and those all work great.
Everything is really based on your final volume which is at, let's say, 70 deg in the fermenter. Think of it this way, you don't really care if you sparge with 2 gal or 2.1 gal. What you care about is getting 5 gal into your fermenter. Run your process once and you'll know what you need next time to correct your assumptions. In the meantime you've simplified your model.
Oh. And your beer is fantastic which is really all that matters.