Correct or not is hard to say. In an amp with no feedback from the speaker output, there is no particular reason to ground the speaker leads for the circuit's operation. You can ground one of them, but if you decide to do it, be sure that you do it properly. Speaker jacks in this kind of amp (i.e. no feedback from the speaker secondary side) can be grounded at will, as long as you don't try to use the chassis for one of the return wires back to the OT. Running speaker currents through the chassis is an open invitation to oscillation. The transformer secondary wires should run to the speaker jack, which can have its bushing grounded. This forces the current that went out of the OT to the speaker to come back into the OT on return without passing through the chassis.