I'm a TT marshall. Good place is where the TV cameras often go. At Appledene (near Greeba castle) just before the marshal scaffolding. A nice wall to rest on & the riders will pass inches from your lens. Public footpath so you're not stuck forever there either, but plan it with a map. Also Cronk-y-voddy straight - very fast & bikes will often overtake down towards the end. At the start of the straight bikes lift off over the slight rise. Once again, you need not be stuck there - with planning & a map.
Use the practise week to try out as many locations as you can. There are many locations to get really great shots.
Also remember the pre-TT on the Billown circuit. Great massed-start racing & great shooting locations.