The connection between gas cylinder and regulator coupler is essentially a flare fitting, meaning it is the face of the stem sealing against the face of a gasket which in turn seals against the face of the tank valve.
Sticking that threaded fitting to the tank valve now means that the regulator stem seals against the face of the fitting - which is not sealed at all to the tank valve - it's just threaded in, with no gasket behind it. And the coupler nut will do nothing to seal that fitting or the stem - that's not it's job. All the nut is supposed to do is draw the regulator stem, gasket, and tank valve together in one big "squish".
Instead of paying $5+ for a bad idea, just buy a freakin' fiber or plastic tank washer and save your gas money...
They give me a couple of new washers every time I swap tanks, but I don't need them. I only use Micromatic regulators now and they have a built in nylon seal that works very well. I haven't used a washer since buying those regulators and have had zero issues.
I have a taprite regulator that I really like but I accidentally pinched the o ring up in the threads and caused it to leak. I am currently using some o rings I got from Lowe's that aren't quite right for the job but if you tighten the hell out of it they hold seal.