Must L + C + R speakers be depth-aligned?

I have two high-end L + R in-wall speakers (that I am planning to mount on-wall in a special enclosure, drivers will sit about 5 inches from the wall).

I am considering buying the matched center speaker (not in-wall). The drivers in the center speaker would sit about 24 inches from the wall.

I understand that in an ideal system, the center speaker's drivers would sit further back than the R + L in a constant-radius arc from the listening positing.

How likely is it that I'll be frustrated with the effects of this? I understand the concept of a timing problem but I don't know what it sounds like. Will I experience a narrow soundstage? Can I remedy this with a surround processor (which I don't own yet)?

Any advice would be appreciated. Here are some details in case you were going to ask:

* Room size/configuration: 13' wide, 11' deep
* Listening position is a couch against the back wall
* I don't have rear speakers yet, those will come later
* Speakers are excellent quality but not extreme - Center used price is ~$1800
I assume you are talking about HT and not music. Your preamp should give you options like individual speaker volume and delay to help with this.
Agree with Zd542 that if this is for HT and you'll be using a SSP I wouldn't think this will be much of an issue. Soundstage width will likely be more a function of the quality of your L/R speakers and the upstream electronics than the placement of the center. If music is also a priority I'd consider getting a good SSP with the latest Audessey room correction to help everything blend more seamlessly. Best of luck.
The common misconception when looking at LCR diagrams is that the C speaker is recessed more. Technically, they are all supposed to be the same distance from the listening seat. When that's not possible, you compensate in your HT pre-pro and set the actual distances in the pre-pro or receiver. That unit then puts in the proper delay to address any issues related to timing.

Most entry-level pre-pros only go in 1 foot increments and higher end ones go in 1/2 foot increments. That's a generalization, of course, but something to keep in mind.
Bad idea to wall mount speakers. You'll get all sorts of reflections from the front wall around the speakers. Better to flush mount.