if i know someone, fine.
if i don't know someone, but have seen them riding ok, and they look like they know what they're doing, fine.
if they're a prick on my commute home trying to get a free ride, it's a no-no, and I will either ride away from them or systematically drop my speed (at which point they usually get aggro and shout at me for doing so).
if they're riding like a prick generally (purely subjective judgement by me), i'll do the above.
If they're a violent little gobshite like some fat little eunoch that tried to tag along my commute and despite repeated requests from me to either back off or go in front of me, got violent when I decided to be very random with my speed – hopefully he's decorating the front of a transit.