GC, are you currently using the boat or has it been sitting up for a while...if using w/ no gas issues, I wouldn't see a need to pull and inspect...however if they've not been in use for a long time, better have a look at them...
I lke to have working gauges...and wouldn't go to the trouble to replace sending units w/ used...go w/ new ones...
Every boat should have a water seperator/ filter unit in the fuel line...if your boat doesn't have on, put one in, no matter what else you do w/ your tanks... ;) :)...this is the one I use:
http://www.westmarine.com/webapp/wcs...llpartial/15/0
You can bring lines from each tank to this one filter, then a single line to the motor...each incoming line needs to have it's own "shut-off" valve...