When the channel doesn't match the native resolution of the TV, something will have to scale it -- either the TiVo or the TV itself. Often one will do a better job than the other and/or handle format changes faster or more cleanly. So usually you want to try it both ways:
Set the TiVo to 1080p only, and move amongst 1080i, 720p and 480i channels, noting especially how the 720 and 480 channels look, and how gracefully it switches.
The do the same with the TiVo outputting all resolutions, making the the TC scale.
Usually you will clearly prefer one or the other.
Bolt 3TB /w Slide Remote, Epson 5020UB, Draper 80" Triggered Screen, TiVo Mini, Oppo BDP103,
Denon AVR1713, DT ProCenter 1000+ProCinema 800's, TSS-SUB4000, 4x Harmony 650