Apparently you haven't seen very many electric toilets...
You should check again ...I have never see any kind of marine head electric or manual without the wet/dry option...
'Cuz very FEW electric MACERATING toilets have a wet/dry option. None that have single motor that powers both the intake and discharge impellers--which are the least expensive and most common and include the entire Jabsco 370xxx series and all the Raritan Sea Versions have a wet/dry option. Except for the high end all china "thrones" the only macerating electric toilets that do/did offer a wet/dry selector are the gawdawful Jabsco 29200 "conversion" and the Wilcox-Crittenden Newport, which had so many problems it was discontinued after only few years.
However, manual toilets to which a motor has been added to WOULD have a wet/dry selector because the motor only replaces the pump HANDLE..the toilet pump is still manual pump. Since those are mostly only found on sailboats, it's possible that's been your only experience with electric toilets. I suspect the toilets on the OP's boat are macerating electric toilets...and to address HIS question:
My aft toilet keeps some water in the bowl after use. The forward head does not. I have to think it's a good thing for the bowl to hold some water in it, yes?
Not necessarily. The joker valve provides all the odor blocker that's needed...which explains why changing it didn't allow water to run into the bowl...it's purpose is to BLOCK backflow.
So my question is: Should the bowl hold some water in it when not in use and if so, why is one toilet not keeping water in the bowl?
If both the toilets are the same model--370xxx series, it's not designed to hold water. Most aren't because water in the bowl will end up all over the head in heavy seas...most people don't want that! The reason the aft toilet holds water is most likely because the discharge line is prob'ly is at least a slight uphill run...a worn out joker valve is simply allowing flush water left in the line to run back into the bowl. Your forward head is working the way it should.
Hope y'all had a good Thanksgiving! Bye again for now...
from the holding tank could get into the boat, yes? I changed the joker valve in the forward toilet with no change. 
