do I go all out and upgrade the trailing arm bushing too?
 

Upgrading the suspension - front is almost done: ER polybronze a arm bushings, monoball strut tops, along with rebuilt calipers, new rotors, new master cylinder, stainless lines, etc. Moving to the back, will do all the same brake renewal along with polybronze spring plate bushings.

With all that, do I go one step further and upgrade the trailing arm bushing too? Car is a 1972 911T so will have to assume the original bushings. I'm almost inclined to just do it since I am doing so much else, but wasn't sure if there would be noticeable improvement.

If I do, having never been that deep into the rear suspension, any more difficult than, say, the front...? Which I also did for the first time...

might as well as long as your this far, hate to find out later that they did need changing

I think I probably knew the answer all along... :-) Do you think that bushing makes a difference? The others are more intuitive to me as having a noticeable effect...

makes a big difference, holding the trailing arms in correct alignment. like putting just the front rear polybronze bushings in and leaving the old front rubber ones alone. But you knew that!!

If you have the spring plate unbolted, you might as well. Another 15 minutes and the arm is out and about an hour to change the bushing (banging out the shell halves is a real pain). It makes a big difference!

For the rear on your car you will have to drop the motor 5 inches or so.

In a word "yes"
~C

What are the symptoms of worn out trailing arm bushings?

Change 'em. If you don't, you'll always wonder why you didn't. You are talking about a small effort with low expense that won't appreciably steepen the slippery slope.