At the risk of seeming to be overly diplomatic, I think it depends upon the type of work you are doing and how much accuracy do you really need. I have a 14 inch Clausing and a 16 inch Monarch lathe and neither are bolted down. I also have a concrete floor that is reasonably level, and generally speaking, I don't work either of these lathes very hard, but then there is always that "special" job that falls outside of my "usual" type of work where I'm either hogging heavy cuts, or where I need extreme (for me) accuracy. I can honestly say that for each of my lathes, I have leveled them each once, and then I check them periodically. Neither has ever moved that I can detect. I would work with it, and if repeatability or accuracy becomes an issue, then I would take the necessary time to level it out and then bolt it down. As with anything else, if you have a problem, you either live with it, or keep going until the problem is resolved to your satisfaction. I do know from experience that a twisted bed will not give repeatable accuracy over the entire length of the bed. This is not a myth, it is a fact. But having said that, most lathes are pretty heavy, and made from subsatantil castings, and due to that, a lathe bed that is actually twisted is somewhat rare assuming that you set it up properly to begin with.
There is no shortage of experts, the trick is knowing which one to listen to!