ya, you can account 5-10lbs of just water weight. i can fluctuate 5lbs easily from water if i have a bad day and don't stay on my intake. i would say a couple pounds from diet and a few from water.
also, why wrist straps? if you use natural grip from the start of your growth you'll naturally develope a strong grip. if you use straps from the start hten you'll struggle once you start deading 300+lbs and it's hard to go back down in weight to develope the grip. it also helps you form better forearms.