In my ongoing 1989 bike rehab, I've got about 3 problems left.  I have a vacuum leak causing the bike to idle lean that I need to track down (WD-40 and inspecting every boot carefully find my problem), a lack of vacuum to the petcock (I'm replacing the vacuum line and cleaning the petcock vacuum port to address this), and my bike will slowly leak gas when left on prime and not running (don't know what to do about this).

Last week I took the bike to the shop to get a new front shoe.  While there, the owner mentioned that he used to have a GS500, and loves them.  I gave him the list of my remaining issues, and he said that the floats will sometimes warp over time (my bike is 20 years old), which will cause the leaking when the bike is in prime.

I have cleaned my carbs, Buddha has cleaned my carbs, set floats, and replaced worn rubber.  Don't know what else to do to fix the flooding when the bike is in prime other than believe the shop owner.  I don't think he's just trying to squeeze money ou of me since he recommended that I handle all of the vacuum leaks myself since tracking them down is easy but time consuming (expensive labor).  What do you guys think?

I can see where, over time, the floats might "warp".  They are plastic and I've found similar things before.

Beyond that, common causes of leaking in the prime position include:

1)  Worn needle
2)  Worn seat
3)  Sticking needle
4)  Sediment from the tank getting stuck between the needle and seat.  (check your tank for rust)
5)  Float height set waaaay out of line

It sounds like the dealer is being up-front with you. 

Floats cant warp ... the floats wear high ... you set it at the gasket level today, in a few months they will be higher ...
Yours didn't seem any worse or any better than the average ... we set em every few months if you wanna be very anal about it. Else ignore it till it starts gushing out.
OK your carbs, I have to recall ... I just didn't think it was anythign of very serious consequence.
Your floats are leaking did I swap O rings ... there is a very important one at the float spigot ... I test for leaks, but sometimes things get knocked about in shipping and can stick open ... Check if its is that ... and O rings ... did I swap em ? you remember ?
Cool.
Buddha.

I did the o-rings about 2 weeks before I sent the carbs to you, so they are less than 3 months old. 

In which case ... where is it leaking from ... exactly pin point it.
Cool.
Buddha.

I believe the fuel is leak is the airbox, but I'll double check tonight.

"the fuel is leak is the airbox ..."
OK then ... stop filling the air box when you get gas. Its supposed to go in the tank ...
Cool.
Buddha.

Sorry about that, I've got loads to do at work and had to write that sentence in two sessions.  Should have proof read. 

The bike is leaking from the airbox, and when it does so, wetness is present in the carb.  It does so in the "On" and "Prime" settings.  The leak isn't much, but it is wasting fuel.