Having your Strats tremolo properly set-up is key to tuning stability. The bad thing about Strats is that they're very sensitive to change. When you change your string gauge you will notice that the trem will no longer be the same... as well as your truss rod... where as guitars without trems just need a simple truss adjustment. Welcome to the family of Strats my friend. You want your trem to sit slightly off the body... this gives it the ability to slightly sharpen notes... instead of just flattening them. If you find yourself using the trem alot I suggest taking it to a tech and having them look at it... and/or explaining it all to you in hands on. Also setting it up for your playing style. If you don't use the trem at all i would suggest putting five srpings on the trem claw and tightening the bridge down so it sits on the body... this gives max tuning stability