should the module depth include the power cable attached? Some cable connectors are 5mm higher than others, so sometimes the resulting total module depth depends on which power cable you use

Good question. Every manufacturer has his own way to measure depth, so I don't think we can set a standard here.
But as you mentioned, since the depth depends on the power cable you use, I would vote to measure the module depth without power cable attached.

In that case it might be useful to also specify for each module whether the power cable connector adds to the depth or not. When planning some cases millimeters are at stake :)

I realize that manufacturers sometimes state some kind of depth for their modules, but I'd rather users measure theirs on their own and edit the module spec here. For example Doepfer says for their A-110 that it's 55mm when in fact it's 60mm and for A-120 they say it's 65mm, but really it's closer to 55mm

I completely understand what you mean.
The problem: since ModularGrid is a Wiki there is no "authority" to validate all the specs, everyone can edit anything, and so you can quite be sure that a lot of specs are simply wrong or entered by different standards. We have this issues on the power requirements too and I doubt that this will ever change.
The quantity of modules in Euro is just overwhelming.

Overall I think the system works nonetheless by relying on some sort of swarm intelligence.
In doubt you can always double check by looking at John's Eurorack Database.

wow great database, thanks for the link

I'm aware anyone can edit anything, I just naively hope people would enter more accurate values if they were told what exactly they should measure :)