You mean for SD content? For standard 4:3 stuff I just leave it Normal (black pillars on the sides). If it's something that's letterboxed for some reason, then I'll switch over to Zoom.
Only a few times a day.Now we are getting spam in here?
From what I've seen on a Tivo HD, it applies to SD sources if you have the TV aspect ratio set to 16:9 and HD sources if you have the TV aspect ratio set to 4:3. I keep my output format set to 1080i fixed and I can change the aspect correction mode on both HD and SD sources by changing the TV aspect ratio. My TV shows a 1080i input at all times for the Tivo.it only applies to SD sources (unless you've set up your TiVo to output SD (only), in which case, it only applies to HD sources... I'm simplifying here).