Note that while people conflate the two and use the terms interchangeably, leveling and setting your Z offset are not necessarily the same thing.

Leveling the bed is a process to ensure that it is square (“trammed,” in many peoples’ parlance) and flat relative to the travel of the print head, preferably at all points on its surface. This may or may not also entail setting your printer’s Z offset as part of the process depending on how its inbuilt process works, or whether or not you do it manually. The point of the bed being level is so that the gap between the nozzle and the build plate is always the same no matter where on the bed you’re printing. Otherwise you’ll have adhesion and gap issues in some places on the bed and not others, etc.

Z offset adjustment is ensuring that the starting position for your printer’s nozzle is the correct distance from the build plate on its first layer. It doesn’t necessarily have anything to do with ensuring that this value is the same everywhere across the build plate.

You should make sure your build plate is level relative to the nozzle first, then mess around with Z offset heights. Theoretically you should only have to level the plate once, unless something works itself loose or some other mechanical issue presents itself. If your bed is not level to begin with you’ll drive yourself nuts adjusting the Z offset because what ought to be the correct value will turn out to be different on different spots on the plate. Your Z offset should be adjusted between prints when you switch from materials that require one temperature versus another, either on the plate or the nozzle itself. (Plate temperatures affect this more than nozzle temperatures, but they both affect it.)

On my machine, a Qidi X-Max 3 running Klipper, I can directly twiddle the Z offset from the printer’s touch screen while printing, or at any time in the console via the Mainsail interface which is accessible via web browser over the network. I have a test print that’s just a 0.2mm thick rectangle I use to assess the correct-ish Z offset, since I can observe it while it prints and diddle with the Z offset setting as it goes until it looks right to me. Since it’s only 0.2mm tall, the generated print will automatically be only one layer thick. I then use that value going forward for whatever that temperature/material is. My printer has a built in routine that allegedly assists you in setting the Z offset but I’ve found that it’s functionally worthless and also much more of a hassle.

Because I’m lazy, I then created a bunch of Klipper macros that do nothing but set the Z offset to one of my figured out values, one each for each material and also for either side of my plate (textured on one side, smooth on the other). To switch offset values I just hit one of those macros from the list in the Mainsail console and it’s done. That way I don’t have to keep a Post-It note or something around with each temperature/material’s Z offset value, nor remember them.

If you’re switching between a flat and textured plate, or the flat and textured sides of your plate, you’ll also have to make a Z offset adjustment. Your nozzle has to be closer to the plate on the textured side to ensure the extruded material makes it all the way down into the valleys in the textured surface. If your printer has a bed probe it will only sense the very top of the textured surface, which probably has a depth of 0.15mm or more all by itself which is nearly the entire thickness of a print layer to begin with.