If you apply the tapes in tried and tested positions by sight alone (rather than by how the nappy feels on you) then you will get the same fit no matter what posture you choose. If you position the tapes according to what feels right, then taping while standing may give you a better fit for standing and likewise for lying down. Around half of my changes are done away from home in washrooms, so they are done standing, by visually positioning the tapes to the guide lines. At home I will experiment more and test the fit of different tape positions, either standing or lying down. I always put my night nappy on lying down, and if someone else is changing me that is also usually done lying down.
Standing to change really benefits from having a wall or surface to lean against to hold the nappy in position while taping. I have a routine that avoids bare skin touching the wall in public bathrooms. Before changing, I use a little pre-cut piece of sticky tape to stick the disposal bag to the wall at waist-height, then lean against that. An alternative that works quite well, and avoids even the need for a surface to lean on, is to use the piece of sticky tape to stick the back of the waistband to my body. This is removed once the nappy is taped up.
I agree that lots of practice and experimentation pays off. A few months of regular wear and you'll be an expert.