I've noticed a lot of the CERTIFIED PSYCHEDELIC CHILLOUT type stuff tends towards similar bpms, and it's sometimes fun to have, say, a 100bpm stretch of tunes, then maybe a few 110 or 120bpms, and a lot of the dubby stuff loiters around 80-90 bpm. It can be a nice challenge to mix 'em for a bit. the 120 bpm stuff usually satisfies the 'restless' chill-out room (read as: 'I'd love to chill out, but I've had far too many stimulants so I'll stand here swaying, nodding and gurning').
Makes it more interesting for DJing and listeners alike (I hope).
As the other folks said though - there really ain't no rules. Which makes it more flexible than the bonkers stuff, where you can get stuck within a 142-148 sort of range - with chill-out you can just 'mingle' your way to a completely different bpm without too much trauma.
I will say it's what CDJs with bpm counters are good for - if you're not one for planning a set down to the details, they give you a chance to flick through a few tunes and see what might go with what. I've got pioneer 100s, so using the 1000s in the chillout at Sunrise in M/cr is a proper luxury for discovering what bpm tunes are (posh that innit - 1000's in the front AND back room :punk: ).