You can if you want, but there's no particular reason to do so. Engine on or off you are flying a TMG.
If you have a TMG rating/privileges on an (A) licence, you log flight time in your power logbook, if you have TMG privileges on an (S) licence log all the hours (including powered) in a glider logbook. If you have both you can log the hours in both logbooks if you want. Remember that TMG hours, even engine off hours, don't count towards the (S) licence recency requirements.