Canon IS Shutter Speed Question

I have these canon lenses that have IS:

EF 24-105mm f/4L IS USM
EF 70-200mm f/2.8L IS II USM
EF 70-300mm f/4-5.6L IS USM
EF 100-400mm f/4.5-5.6L IS USM

With IS on mode 1, how low of a shutter speed do you feel comfortable shooting hand-held? I've heard that you should use the focal length to determine the shutter speed, like if it's at 400mm, then you need to use at least 1/400, is that true? Also, how does IS affect that?

Thanks!