When using my rotozip I found it far easier to control when I cut in a counter clockwise direction and pulling it towards me using both hands to secure it.... and only in a horizontal plane!
For instance, after marking the window opening, I would start at the farthest point I could reach from me, and with both hands securely holding it, bring the rotozip towards me making sure any curves to be cut are cut counter clock wise.
Hope this makes sense. Give it a try on scrap lumber.