Want to start an argument? Ask several grapplers whether or not you should cross your ankles when doing the basic armbar.
Many people will definitively tell you that you should NEVER cross your ankles. According to this school of thought your legs should function as ‘independent suspension’, and that crossing your ankles makes it harder to put pressure on your opponent’s face with your top leg and squeeze his arms with your inner thighs (sample armbar with uncrossed ankles).
Other people will tell you the exact opposite, that crossing your ankles gives you MORE control over your opponent, and makes the armbar harder to escape (armbar with crossed ankles). These people will tell you that crossed ankles allow you to control the far arm and shoulder more easily, and make it harder for your opponent to slip or rotate out of your lock.
Just to make matters more confusing, I have sometimes heard people say that crossed ankles are wrong, but then these same people go ahead and use both methods (crossed and uncrossed) while sparring or competing. Likely they were just instinctively reacting to whatever their opponent was giving them, using the leg position that was appropriate at the time.
Personally I use both methods, and I’m never quite sure which I’m going to use until I feel my opponent’s energy. In the end I suggest that you experiment with both methods, and see if one of the methods is more applicable to your body type and armbarring style.