I deliberately come under a little so I can top the kettle off after flame out to my exact volume, 5.5 gallons. This also drops the wort a few degrees so it's a little less work on the chiller.
If you try to top off your fermenter to 5 gallons you may need to account for any volume left in the kettle. If you left about 2 quarts, back it out along with an additional 25% because it was really a concentrated wort.