We have both source selectors set to C, and the favoured autopilot to use is the R. If we were to use the C, the F/D's would blank as described above. I know most 75 operators use L source on the left side and R source on the right side, so using the C autopilot prevents the F/D's on either side from blanking.
We are not supposed to manually tune VOR's in the FMC when the L autopilot is engaged as "dual FMC reboot may result" (!). That probably depends on your FMC fit though.
Our order of preference for the A/Ps is R, L, C on the 757.
On the 767, the L or R may be used as a first preference, the source selectors are the same as for the 757, both in C.