I use Beersmith, which gives me a good starting point for how much water I'll need. Various environmental conditions usually change my boiloff, and hop amounts/types might skew trub absorption.
When lautering, I also check the gravity periodically. I agree with Piratwolf in that I'd rather get the beer I want, even if it's only 4 gallons. To help with this, I only sparge until gravity nears 1.010, at which point the grain is done. This has yet to be a problem, i.e. I hit my desired water volume before that point is reached.
Also of note, I normally shoot for my ABV to be a tad higher than I may need/care about - 6.5%, for example. ABV being obviously linked to OG, so if I decide I want an extra half gallon of beer, I'll do a quick calculation with Brewzor, and decide if I can dump a half gallon of water in. Normally I'm OK with it.