I have drafted people I don't know on a couple occasions, but I always ask if it's OK with them to be on their wheel before doing some good ol' wheel suck.
If they don't know what it means when I ask "can I draft you?", then it's safer for you and for them if you don't do it.
Just my 2-cents.
I think I should start a new thread entitled: "I hate how much I hate being passed". Even today, when I mounted my Nokian Extreme on the front (which basically means I go 3 to 5 MPH slower than with my slicks); when someone with slicks passed me on the trail, it irked me to no end.