I did a little tranny write up here including the drain plug http://tinyurl.com/m5hn5c
Not necessary to weld, if you have a mig handy, wouldn't hurt to tack on the inside nut.
Don't use dexron, ATF +4 only.

You ought to do it, if nothing else, filters have limited life spans, and I know you'd kick yourself if you fried your trans due to a clogged filter.
A good time also to look for any metal shavings that could by symptoms of a looming problem.