If you're looking to fish mostly treble-hook topwater baits like the Whopper Plopper 130, MS Mini Slammer, BBZ 1 Rat etc. is it better to have a shorter rod like say a 6'6" or would a longer rod like a 7' - 7'6" be a better choice?

I use a shorter rod if the bait requires some sort of twitch, stroke, or popping action that requires snapping the tip down.  For what you listed, a longer rod should work fine.

1 minute ago, J Francho said:

I use a shorter rod if the bait requires some sort of twitch, stroke, or popping action that requires snapping the tip down.

So a shorter rod would be better for stuff like spooks, Sammy's, Evergreen SB's and basically anything that you would want to "walk the dog" with?

You got it!

I find that a 7 give me the best of both worlds.

I actually prefer a bit more distance over accuracy.

 

The extra length also make it a bit more multi purchase 

Personally I would say 6-7 ft. It all depends on personally preference.

I agree.  I don't like rods over 7 foot in general but for top water, I use a 6-10 duckett white ice for buzzbaits and a 6-6 or 7 foot IMX MBR for spooks and poppers.