With any of the vintage Hondas, the "profile" of the front and rear tires must be the same type, or handling will have oddities.
For example, on a CB750 K0-K5, the stock front 3.25x19 matches the rear 4.00x18. If you substitue a low-profile rear tire like a 4.00-85x18, the rear sidewall will "lean over' at a different rate from the front: specifically, it will lower the rear axle centerline about .250" (1/4") compared to the front tire at full lean angle. This causes an increase in trail as the bike leans over, which makes the rear feel like it is "washing out" from underneath you. In contrast, increasing the rear tire size to a 130/90 will cause the rear to "rise up" at the axles centerline during this lean, which reduces trail. This makes the front end feel like it is trying to turn wider than you expected, and you end up re-correcting as soon as the turn starts, then re-re-correcting that correction, etc...
Either situation cuases you to feel insecure as you dive into turns. In stock tires, there are a couple of other concerns: edge traction and vertical load capability vs. side load capability.
A round-profile tire has a very even load transfer rating when going from vertical to angled, but the contact patch shrinks slightly on round tires at lean, because the tire rolls away from the rim's edge. The trigonometric tires (like TT100 profile) compensate for this by having incredibly strong beads and a smaller tire patch surface while vertical, so the G-forces are consistent all the time (important especially when swapping from one lean to the other at high speed, where G-forces come into play at the vertical point). BUT, "trigo" tires carry less weight for a given size, and require considerably higher tire pressures.
The shape of the tire pattern at the edges has more to do with side-loading than you might think. If there are "open vertical crossbars", i.e., openings that go STRAIGHT ACROSS the tire from the edge inward, there will be a "drift" capability. This accomplishes 2 things: 1.) it lets the tire "warn" you that the edge of traction is approaching by drifting slightly off of the anticipated line and 2.) it increases the forward drive and rearward braking traction by applying sideways edges to the concrete, relative to the bike's direction. Some tires will angle these "open vertical crossbars" to reduce the drift, but this also reduces corner brake/drive traction and grip, and removes the "warning" that you are almost out of sideways traction. The CB750 has more lean angle than most bikes (MUCH more than, say, a 500/550), so sideways traction issues are important on these heavier bikes.
Front tires' tread pattern plays another role: directional stability. If the grooves go mostly AROUND the tire's circumference, it will be more stable in the pointed direction than if a block-profile tire is used. But, it will have less braking traction, because the block profile puts more "edge to the concrete" whne the binders are on.
Lastly, size makes a large difference to handling. If the front tire is larger than stock and the rear is stock (or smaller than stock), the bike will have more trail and the handling will slow down. If the front is smaller than stock and the rear is stock (or larger than stock), then trail is reduced and the bike tends to become "twitchy" and very quick-steering. At the extreme, the latter causes wobbling and a tendency to track every groove in the road.
So, for best all-around results, keep the sizes near to stock and match the profiles front and rear. If you switch to metric, match the second number (profile number) to the original for best all-around results, and use the SAME profile front and rear (like 110/90 front, 120/90 rear). For hard cornering, switch both to trigonometric: but remember, for example, the 3.25 round profile equals the 4.10 trigo profile: that's an equivalent switch. If you undersize the tires, or use trigo tires, raise tire pressures. If you oversize the tires, you must experiment to find the "sweet spot" where the pressures don't change too much from tire flex.
A trick to remember from Continental Tire's motorcycle tire engineer: measure your tire pressure cold, then ride for at least 10 miles until the tire is well warmed, then measure the pressure again. If it rises 2 PSI: you're spot on your required pressure. If it rises more than 2 PSI, your PSI is too low. If it does not rise 2 PSI, your PSI is too high, and your tire patch size will be reduced. When riding 2-up or heavily loaded, add 2 PSI to your "normal" PSI as a start, but check again when warmed up.
As a reference: I run my (CB750) TT100 (4.10x19 front, 4.75x18 rear) tires at 34 PSI single-rider. I run my Avon touring tires (120/90 rear, 110/90 front) at 30 PSI.
Update for Avons after 2008: the Avon tires now use 35 PSI as their minimum, and I am running my new ones at 38 PSI front, 38-40 PSI rear, to make the corners smoother and ore 'even' feeling.