Depends. The FAA has held in the past that logged time can be considered compensation and used as the basis for various violations (commercial flying with no CPL, flight time limits, part 135 violations, etc) in the case of pilots who have an obvious time-building need.

If you're a gainfully employed turbine pilot with an ATP I suspect they wouldn't bother since you don't really need any more piston time, but a low-time professional pilot could get burned.

I think some occasional CFI work could be considered as recreational if you're not in time building mode. But it should probably be "ad hoc" with friends/family, as opposed to standing around the FBO offering free instruction to all comers...