Why dont you half fill the bath, then you cant go wrong
I was taught that Silicon is not good under compression, it was designed to stretch. As such I never fill baths prior to Silicon, the stretching abilities of Silicon should be sufficient to accomodate movement of a millimetre or two easily. If a bath moves more than that I think there are other issues to address and whether the bath was full or not I don't think it would make a lot of difference.
swe !! most baths require the tile to finish on top of the bath , so it is required to be in situ , unless it is a stand alone.I always remove the tub before tiling myself. Same goes for sink and loo and pipes and whatever.