I'd absolutely ask before drafting, especially if the other person is riding solo. There's a decent chance they haven't ever had someone draft them before, aren't great at holding their line, don't know to call out road hazards, may swerve wildly when they go to do things like take a drink from their water bottle, etc. Also, they may not even know you are there if you don't speak up. Most of the time I don't wear a helmet mirror and have had people right on my wheel without even knowing it. That used to freak me out a LOT when I was a newbie, because I know when I get tired I can't hold my line at all and I don't want to make them crash.

Corsair is right on with the advice to soft pedal, or to pedal with very light resistance while keeping your same cadence. Here's a great article on riding in a paceline:

http://www.gccycling.org/download/paceline.shtml