I have a 24" cabinet for the microdrawer and all drawer bases of different sizes. (The corner next to the hob is a door made to look like drawers - we have three pulls on it - so that the flow of the bases goes all the way around the kitchen. It houses a magic corner unit.)

Here's a picture of what it looks like:

You need to decide what look you want. You'll have more storage (in the drawer next to the microwave) if you have 24" and 30" though and for us storage was a priority.

Not quite sure if this is what you're looking for, but this is a 27" cabinet waiting for a 24" MW (old pic). The fridge and cabinet above the fridge are 33" and the base cabinet next to it is 12", and it doesn't bother me to have 3 cabinets in 3 different widths next to each other.

BTW, the 24" MW ended up being 1/16" too big for the space, so I ended up with a 22" MW.

Thanks for all the pictures and advice guys. You all have such great looking spaces and seeing the cabinets in different widths next to one another really helps. I think I'm leaning towards having the microwave be flush (width-wise) with the drawer below and having a 30" cabinet next to it. Better go call the kitchen designer! Thanks again for the help.

I think you would be fine having a 24" drawer MW in a 27in cab. You are only talking about 1 1/2 inches on either side of the MW. It won't look odd.

No question - put the 24" microwave drawer in a 24" cabinet. No reason to waste 3" for nothing, especially when varying width cabinets actually look better than having all cabinets the same width.

If for some reason width symmetry is a must in your view, throw in a 6" pullout in between two 24" cabinets.

But assuming you're trying to squeeze in as much usable space possible, I'd avoid the 3" waste.