You take your kart to the track “baselined” or at the track you put it to what ever “baseline” you use. You can baseline your kart to your best guess starting point, either on or off the stand. You can do it either way so long as the results are the same every time. If results change when your baselining, either something got bent, worn out.
After that it’s all about making changes to fix on track problems. If you going to make a mechanical change logically needed to try to fix an on track problem, it isn’t going to make any difference weather you make the change on the stand, on the ground and weather the driver is in or out of the kart. That is so long as you can isolate what ever mechanical change your making, from outside influence. That means if your going to change something at the right front corner, put something under the right front corner so there will not be any binds on what your mechanically doing. Putting the kart on the stand will help keep outside influences from effecting your mechanical doings. It’s about how good a mechanic you are when you make the change and what mechanical considerations you automatically include when you work on things mechanical.
In a nut shell, it doesn’t matter one bit so long as your not a poorly skilled sloppy mechanic.