Regarding changing the tensioner - both ways work. Pushing it in decompressed is not that difficult. I oiled mine both installing.
Regarding the rails - I didn't like the noise I was hearing so I changed the tensioner. Wasn't crazy about the way the timing sprockets looked and had an oil leak so I changed the chain. Figured if I was going to take it apart I might as well have the rails just in case. The two side rails had virtually no wear, the bottom rail fell apart in my hand, and the upper rail had some wear.
The directions on the link provided above to Bredan Purcel``s page are very good. I followed them and was surprised at how well and relatively quickly it went. The engine is still noisy, but quieter and the sewing machine sound has gone. Plus no oil leaks!!

The car is a 91 318 and has just under 120K miles.