Topping up boil volume with water?

Heya,
I was trying to mash/sparge ending up with 6 gallons of wort to compensate for evaporation in the boil.
I've just run out of sparge water leaving me with 5 gallons of wort. I don't really want to dilute the wort after the boil, so can I just add water as it boils off? Surely only the water will evaporate anyway? and as my SG of the 5-gallons I collected is reasonably on target, it shouldn't matter, right?
Is there any reason why I should worry about adding water to the boil to compensate for evaporation?

thanks,
H

Can you heat up another gallon of water?

If you add water before or after the boil to get your final 5 gallon amount you will end up with the same beer. If you boil down 6 gallons to 5 or if you boil 5 down to 4 and add 1 gallon you should end up with the same S.G.

If the 5 gallons you have preboil is on target for your gravity, it's fine to top it up during the boil.

It will effect hop utilization slightly. The larger the boil volume, typically, the better the hop utilization. That may not be a factor for you, though.