A reader writes in the comments:
"In regards to the Rehearsal dinner, I am in a unique situation. Every one of my guests is from out of town.
"None of my bride's guests are from out of town.
"I know it is traditional to invite all out of town guests to the Rehearsal dinner, but what is the SOP for this situation?? How will it look to her family if every single one of my guests are invited to the rehearsal dinner but none of hers?"
It would look weird. It would look like she's the visitor in a strange, hostile environment. In fact, it would sort of look like the end of Rocky IV, where your fiancée is in Russia and the whole crowd is Russian and rooting for you, Dolph Lundgren. (After that, the metaphor sort of falls apart. Unless she kicks your ass, holds up an American flag, and screams, "If I can change, and you can change, then we all can change!!!!!")
Avoid that scenario.
Let's look at the fine print of rehearsal dinner etiquette. The invite list actually includes three categories:
1) Out of town guests
2) The wedding party
3) Close family friends
This third category is your loophole. And this category is fungible. There's no hard rule on what qualifies as a "close family friend," so you can pretty much invite whoever you damn well please.
Even if your parents are footing the bill for the rehearsal dinner, they should understand. This is a pretty reasonable request. A 90/10 split is hardly fair.
And it doesn't have to be 50/50. There's no need to keep score. But yes, your instincts are correct, make sure she invites a smattering of her guests so that it won't feel so lopsided. (If possible, go for the older "family friends," which mitigate the chance of jealousy from your actual friend-friends.
For our more expansive thoughts on the rehearsal dinner, see: Staying out of Hell: The Rehearsal Dinner.