Putting an "is on screen" check above the collisions and patrolling could help, if done right.
I wouldn't bother with blank sprites though. Don't worry about that. Unless your image memory use is getting to the point of extremes, there's no point in trying things that may even make performance worse if not done correctly.
As always, organize everything into "groups" and use the debugger to see if it's actually helping performance. That's what the "profile" tab is for. It shows how much CPU resources something is using.