Do I need to use plumber's tape on bath tap threads?

Perhaps.

Dunno what that is, but the taps are sealed using a fibre washer, which should be all you need.

Not really, the seal is made between the tap and the tap connector by the fibre washer - it might be worth replacing the washers though:

It isn't a hard job - just fiddly - certainly requires no skill.

No, the tap unions should have fibre or copper washers which seal against the end of the threaded portion of the taps - so the threads themselves don't have to be sealed.

Get the tool, then do as I did - cut the tommy-bar off and bash a 12mm socket over the end (don't do what I then tried - and failed - drilling a hole through the pair to fit a cotter pin)

Then get an extension bar and a decent ratchet, and then, finally, you might have a half decent tool.

(Sorry, just spent last weekend doing the same bloody pain-in-the-arse (and elbows) job)

You certainly can, but it's probably not needed or especially useful.

There are two sorts of thread: parallel and tapered. Both work in the same way - they use the lengthways wedging action of a screwthread to squash some sort of compressible gasket. The difference is that a tapered thread can seal against the thread itself, but the parallel thread must seal against some separate flat radial face instead - either the end of the threaded tube (inside the thread) or some sort of flat-faced nut (outside the thread). Usually a tap will seal with a small washer on the end of the threaded tube, a bulkhead connector in a cistern (toilet or loft) will use the nut and external washer.

On a tapered thread there's obviously no washer (and in fact an added washer will be no benefit at all). However threads alone aren't a perfect match, so thin PTFE tape is a useful sealing method.

On the parallel thread, you need the washer to seal. Stick what you like on the threads, they'll just never get the compressive force to make a good seal there. The same principle applies to compression fittings - it's the olive that matters, not the nut.

You _might_ use PTFE tape on a parallel thread as an anti-seize mechanism, but it's likely to be scrubbed off as much as it stays behind.