Matching timbre is important... but
I agree that matching timber is important and desireable. But, this is a built in system. Aesthetics, room shape, and most importantly Wife Acceptance Factor could be in play.
I worry most about timbre in my LeftCenterRight. If I'm going to give up a match, I'd doing it with surrounds.
jpaul, they are right about timbre. It's good to have all the speakers come from the same manufacturer, using the same parts, design characteristics, materials, engineering, etc. Frequently, center and front speakers are designed to work with each other, and then surrounds will be designed to work with those. But you can't hang a bookshelf from the ceiling, can you?
Sticking with a 5.1 isn't a bad idea either, but why not just go for it? Put the ceiling speakers as close as possible to the dimensions in the audioholics links, but line them up to the lights aesthetically first. George Lucas would understand. We don't live in a perfect world.
Buiild back boxes for the ceiling speakers... any speaker manufacturer could provide specs on cubic feet of enclosure. PM me if you need help, or post publicly to get more info. You're looking at about .75 cubic feet for a 5.25 to 6.5" driver. Make sure any enclosures are matched in size and shape as best as possible.
It's not hard. Seal it with caulk (airtight) and insulate the enclosures before sheetrock. Use heavy and dense material, 2x4 and 7/8" ply (at least).
My compadres have made good points, but I'm on the Go For It platform this election season.