I try to keep the ISO at 100 for a lot of my shooting and will adjust it to 200 and sometimes 400, but rarely above that unless I'm really pushed during low light levels and handholding or on the shoulder brace and steadicam. On the tripod I rarely go above 200 ISO.
Canon's system of bringing auto ISO into play during AV/TV semi-auto modes is a bit of a pain (I hate to see ISO showing 4-figure numbers on the rear screen live-view) It is not so bad in bright light conditions beacause ISO remains at decent levels, but as soon as the light wanes I like to switch back to full Manual and keep the ISO dialed right down. I'll remove the PL filter or ND, then open up the fast aperture lenses a bit more if possible, just to keep ISO values down. That we are now able to control ISO, shutter, aperture and sound levels in the 5D is a big plus factor.

I too generally use a fixed & lowest possible ISO but the Canon Auto-ISO setting is actually pretty useful. It is smart as changes are 'damped' & the values don't fly up & down as the light changes. In manual mode just try setting your shutter & aperture & them with Auto-ISO on slowly pan around a room from dark corner to bright window & see the exposure change smoothly & gently without any of the wild 'pumping' that I have seen with auto-gain on proper video cameras.

Indoors I would generally use Manual all the time, or when in my favourite AV mode I'd set the chosen aperture, point at a mid-tone, and then press the exposure lock button.

Outdoors in changeable scenes with variable light levels during a pan or non-stop clip, then I'd do the same and be in full control.

When I'm not run-n'-gun and doing tripod work at a more sedate pace, I'll stick to Manual and also manually adjust the ISO to the lowest possible level.

When filming the exact same outdoor scene in TV mode the camera's auto system would close the aperture far too much even before you try to lock the reading, and ISO levels would also generally begin at higher levels than in AV. This is just one of the reasons why I rarely use TV mode on the dial.