Some people worry about infection with their water, I don't (mine is awesome), but I hear it a lot. When I used to extract, I would only boil 2.5 gallons then top up to help drop the temp and get the right amount. Its not something I like to do with full boils, its just me. When you are boiling a full batch you are sterilizing the entire batch and creating a full flavor which is going to change when you dilute it. Its just not a good practice if you have the right equipment. If you get 4 gallons on this batch, increase your boil size or boil time to land at 5 gallons. Its not written in stone, obviously, but its not something I would do.
Every brewer has their own practice, I will never dilute my all-grain beers - others might. I take good notes and adjust.