I remember from editing quake 3 content that they say that a 256 x 512 was just as expensive as a 512 x512... Is this true for all games or was it just true for their game
It's good to maintain a rough pixel density, but you can fudge a little in between. You just don't want to texture 2 10' objects, one with a 128, and one with a 512. A little discrepancy is fine, and you'd be better off filling up that sheet.