For general industry, OSHA says that as long as the cylinders are “connected for use,” (i.e., the cylinders have the regulators and hose attached) then they may stay on the dolly.
This storage practice may not be used in construction, however. If gas is not to be drawn for 24 hours, the welding/cutting outfit must be broken down and the cylinders put into storage. OSHA says that this is due to the rougher nature of most construction sites.