Howdy
Yes, the strain screw on a S&W is supposed to be tightened all the way. Yes, you can back it out about a half turn or so, even a full turn, nothing bad will happen. I have done it on many of my Smiths.
However, if you back the screw out too far, the spring will not maintain the shape needed for clearance in the grip. Back the screw out too far and the spring will begin to jam against the frame as the hammer is cocked. This is very easy to see. Just remove the grips, and back the screw out a bit, and cock the hammer. Keep backing the screw out and cocking the hammer and eventually you will see the spring begin to bind.
You can buy an after market spring from Wolff springs that is lighter than the factory spring.
http://www.gunsprings.com/index.cfm?page=items&cID=3&mID=58
But don't forget, when you pull the trigger on a S&W you are also compressing the rebound slide return spring. Probably at least half of the resistance you feel is from compressing that spring. You can buy a kit from Wolff that contains both a reduced power main spring and a reduced power rebound return spring. If it is an older gun, just taking it apart and removing old, hardened oil will also help reduce friction and reduce trigger pull. Really, you ain't going to get much better reduction of friction with an older Smith by polishing anything, they left the factory with a fantastic trigger pull. I can't say anything about the newer MIM Smiths, I have not been inside one. But putting in a reduced power rebound slide spring will help reduce DA trigger pull without causing any light primer hits.
Of course, you need to know what you are doing if you want to take a Smith apart without springs flying all over the place. This book will show you how to do it. It is well worth the price. It is simply the best book on the market about working on Smiths
http://www.gunbooks.com/sw.html
Midway has it in stock. Or buy it from the website I posted and all the money goes to the author, not to a middleman.