No. Many divers use what they have available to start with. Many carry their personal preferences from backmount into sidemount. Others intentionally experiment with different cylinders (depending on what dive buddies / friends may happen to have) until they come across something that they really like.
For example: I happen to like AL80s for recreational sidemount - I had two sets of backmount, manifolded double AL80s, and broke one down for SM. That way I worked out the trim and weight, so when I travel and (in the Western hemisphere) encounter the ubiquitously available AL80s, I have some idea of what I need. My favorite single cylinder is a HP100, and my favorite 'somewhat more gas' backmount, manifolded double cylinders are HP100s, so I dive a pair of HP100s for SM as well. And, I had available a set of manifolded doublke HP120s which I really didn't like for BM, so I broke them down for SM, and really like them. There is a common thread here, as well - 7.25" cylinders. I tried some 8" cylinders diving SM, and they are not my preference for SM.
Others on SB sing the praises of LP steels for SM. It is really a matter of both personal preference, and the requirements of the dives you are doing.
