We are installing the apron to a corner whirlpool tub, but it needs to be trimmed. What is the best way to trim fiberglass and what do we use? Thanks.

How much of a cut needs to be made. Your worst enemy in cutting it is vibration. That can caus spiderweb cracks to appear. How thick is the portion you want to cut through?

I mostly use a side grinder,with a very thin 'cut-off' blade,it's a small hand held grinder.

You can also use a Dremmel(sp) tool, spin cutting can go off line easily though.

Can I use a table saw with a carbide tip blade... or another?
I am cutting through a piece about 3/16" thick.

This is a flat panel?

Yeah,I have used a table saw for things like this,namely 1/4" plexiglass,but even then,it wants to crack at the edges, like califdadof3 said,vibration is the enemy, making a smooth pass through the saw can work,but you have to take care not to hit the blade AFTER the cut,on it's up-stroke.

Another method is to use a roto tool or a router. And clamp a board to the surface to act as your guide.