It depends to some degree on your build. For me, I have more of a bodybuilder's physique, and regardless of what size suit I buy, enough proportions are going to be off that it's almost beyond help from a tailor. So, for me, MTM is the way to go.
For others who have a closer to average build, an off-the-rack suit will usually work well, but yes you should have it tailored.
FWIW, I own one suit from Indochino and one from BlackLapel, which are both your entry-level type MTM suit company. The fit is superb, and something I have never been able to achieve with off the rack + tailoring. The fabric is ok, but nothing high end. For me, though, fit is much much more important than quality (within reason).