When I formulate my recipes with Beersmith I always do them for 5.5 gallons because the extra 1/2 gallon makes up for the amount I lose to the trub at the end when racking to bottle. Before I started doing this I was ending up with around 43 beers rather than 2 cases.
Adding water will lower OG and can make the final product taste a bit watered down, believe me I found out the hard way a few times.
Once your figure out your efficiency (if you have yet to do so) and have any brew software, just formulate everything for 1/2 gallon more to make up the difference.
Primary 1: Winter Tropics Pale Ale
Primary 2: Appy IPA
Primary 3: Truly Triple Hopped IPA
Secondary 1: Air
Secondary 2: Air
Bottled: Edworts IPA, Nierra Sevada, Stork Blue Celebration Ale
On Deck: Maroon Effect Amber, Blonde, IPA, APA