if you don't cls and don't flip then the screen will stay the same anyway.
This isn't possible on my computer. (Is it the same on all macs/minimacs?)
The screen flickers, and old graphics that got overdrawn is sometimes visible.
(Edit: but only in fullscreen)
My method is a bit different. I alsways separate logic and drawing in my code, and when the game is paused it doesn't run the logic part. Like so:
while not gameover if not paused run_logic() endif draw_graphics() wend