Specific classes might not emphasize it, but on the art of BJJ as a whole certainly does emphasize flexibility. I'm amazed at some of positions my teacher gets his body into, they look like a combination of a pretzel, a wacked out yoga posture and break dancing.
However, unlike some other m.a., you don't generally see static streching at the start of class as a 'warmup' because there is a growing amount of research suggesting that this negatively effects training, making you more succeptable to injury, or, at the least, reduced strength. However, a lot of classes do emphasize warmup movements that double as movement prep, which serves to warm the body up, lubricate the joints, and increse blood flow to the muscles, without the disadvantages of static streaching. The best time to do static streaches is about five minutes after working out (though there are different theries). This is because the muscles are already warm. You wait five min. so that you can regain you're focus and you don't run through the streaches with bad form (which can be bad for you in the long run). Think in terms of streaching an elastic band, what streches better and is less likely to break, a cold band or a warm one?
Anyway, more specifically, a lot of high level BJJ folks talk about the importance of flexibility in BJJ, check youtube for vids of Bravo, or Rickson, or type in gymnastica naturale (which is sport specific to BJJ). A lot of BJJ folks incorporate yoga into their training. Here's an article on that:http://www.grapplearts.com/Yoga-For-BJJ.htm
Anyway, the main thing to come away with here, and this is an important point to get out there as for as I'm concerning, is that static streching before intense exercise will make the body MORE prone to injury, not less.