If your K8 cafe' is intended to handle well, there are 2 things to consider with wheels:
1. The front trail was designed with 19" wheel + 3.25" tire, 17" wheel + 4.50" rear setup. Simply changing to an 18" front will reduce trail (which is already relatively short on the CB750), which will make a choppier ride (due to quicker steering) and significantly reduce stability above 60 MPH.
2. The rear wheel was made 17" for mostly 1970s rubber reasons. The 750K0-6 always wore out the rear Bidgestones quickly (4000 miles), and the K8 had even more HP: Honda decided on a bigger tire to try to extend their life. Today, this is not so much of an issue: modern Avons and Metzlers can deliver 20K+ miles in the same setup. So, swapping to an 18" rear wheel, which will greatly improve your tire options over the 17" size, then allows use of the modern metrics (110/90x18 for best handling, 120/90x18 for a slightly larger footprint if the engine is 80 HP or more). Use the matching 110/90x19 front (it will just barely squeeze in on a 1-disc front end) for the most trail improvement and "evenness" when tossing it into and over the curves. A 11/90x19 front is acceptable, and usually the only option if the front is dual discs, requiring deflation to be mounted, inflation in place.
If you're just going for looks, do what you like.