I suppose it matters if you are either just using the machine at home, or operating it at a location for money. If you are running the machine home use only, you don't really need mylar to protect from dirt/grime...because you should already be cleaning the playfield every once in a while. Also might factor in what era your pin is from (ie. Diamond Plated playfield or not, in which case, your BSD is diamond plated).

Pros of Mylar:
Mylar protects your investment, namely your playfield. I use mylar on my playfield on places where I know it will get an awful lot of wear. What that particular mylar spot is, depends on your game. I have the Inner Sanctum on my Shadow mylar'd, as well as the VUK drop off point on Bigfoot Bluff on my White Water. The ball tends to make nasty dime sized divets in there, so the mylar acts as a sort of barrier so the ball doesn't screw up my playfields more.

Cons of Mylar:
The downside to having mylar is dirt and junk just tends to build on top of it (makes it look like a dark-murky smoke is hovering over the playfield surface), but its hardly noticeable if you run the game for home use.

If there is a good thing about not having mylar, is you can really see the beautiful shine/reflection off of the playfield when its waxed/cleaned.

Putting mylar on the flipper button area of your TSPP, is probably a good idea. Mylar around the buttons, and then place the side armor. You can never be too sure with the crappy Stern sidearts that fade easily.

Cool deal. Have fun with your BSD. Post some pics of that bad boy.