Dead wrong! The axle must be allowed to freely move while tightening the axle bolt so all the bearings' inner races and the distance collars are in contact, tightly torqued together.
This is important. If you don't follow what John is saying then discard whatever you knew from any previous bikes you have owned, and re-read it. My experience with other bikes was that the spacer simply floats between the bearings - whether this was an incorrect assumption on my part, I don't know and can't remember.
Whatever - it is definitely not the case with the ST1100 and ST1300. The axle clamps everything together from the properly torqued nut - remembering of course to install the stopper bolt before torqueing up the rear axle.
Re the ABS sensor - The manual that I have access to says not to remove the ABS sensor when removing the rear wheel. If it is removed and left loose, it can be easy to forget about it when manoeuvring the heavy wheel. If it is left in the bracket, you know exactly where it is. It will come to no harm when removing the wheel. Just be aware of it when putting the wheel back. Before taking the wheel off, find a piece of wood or something that will just fit under the rear wheel, and then remove it to one side. When you put the wheel back in, you can rest the wheel on this wood and it will be at the correct height to slide onto the splines.
The sensor is a close fit inside the bracket, and corrosion can cause it to corrode and seize, so on another occasion, it's a pretty good idea to take out the sensor, clean it up and re-grease the body (not the end magnetic surface). However, if it is stuck, do not attempt to remove it until you have stocked up on sufficient swear words. You will need all of them when that very gently tap to pry the sensor flange from the bracket results in the bracket from being snapped off the inner shell of the final drive. It only takes a very tiny tap to break the cast alloy. Please don't ask me how I know this. No. Please don't.