In theory, mowing on flat, even terrain- you can mow a straight line at a reasonable speed.
I have an old Toro Groundsmaster with rear steering. I have found that any slight plat in the wheel translates into a noticeable drift in the back. Also, any little bump that you may encounter, even if not noticeable- may cause you to shift the weight forward and that momentary decrease in ground pressure will cause drift.
Thankfully, I was given this mower as a "project", so I have little money in it. Otherwise I would have shot myself several times. Rear steering is really best suited for flat, even ground. Even a gradual hill can be a problem for me.
If the terrain is not the issue, and the adjustments are correct, then probably practice will be the best help. Rear steering is defiantly soomething that takes getting used to.