hey dude. well it is not entirely necessary to route a cavity to install a floyd rose into your strat. however, not doing so will prevent the floyd from being able to pull up. you will still be able to do dive bombs. and also......your guitar might have a ridiculously high action.
for me, i had my strat routed for a gotoh floyd. it went well for me, no alignment or anything went wrong. and i'm really happy with it.
so the answer to your question is "no you do not have to route away to install a floyd rose".......but........is it advisable NOT to route. i wouldn't advise it personally.
and also......its not cheap......so unless you're very sure you wanna do it to that guitar, and that you really want a floyd rose on it. if not dont do it.