My school doesn't train flexibility formally, but we spend about 10 minutes stretching usually, and our instructor says you should stretch at home A LOT. He recently gave a speech abut how a certain motion is one of the most important in BJJ, and it is all based on flexibility. The motion is when someone is passing your 1/2 or open guard by standing and trapping one of your legs between his. He showed how you wrap the other leg around and inside his armpit to un-kill your trapped leg (similar to Roleta-guard), then post your now un-trapped foot on the hip to recover full open guard. I hope that makes any sense at all