There have been a number of fires atributed to overheating of ceiling mounted ventilation fans (absence or failure of thermal cut out) which has then fallen from it's mounting and resulted in the ignition of composite polymer bath tub.
off set position away fromtub may be the way to go.
Also as Tom states, candles are responsible for a considerable number of domestic fires. (Less likely in shower cubicle though) unless devil worshippers

Similar situation may apply with regard to ventilator overheating and ignition of composite polymer shower tray (HRR greater than plastic)
BTW........Live in a four storey Gaff with protected staircase and self closing 30 min doors on my bathrooms

Handy as I dont fancy abseiling from either of my balconies as a MOE with Mrs Bleve (in accordance with the Marriage Act) and sprogs in tow.......Great view though