Angle Water Stop riser to sink

At the inlet side of the riser connection on an anlgle stop, using 3/8" white zurn pex pipe to the faucet. I have been told by different plumbers that a copper insert into the pipe is required to stop crushing of the pipe and by other plumbers I have been told that as long as you use a nylon compression ring you do not need the insert. I see from one manufacturer that if the pex is 3/8 or greater then the insert is required, but not 3/8" or less. Now, when I install the nylon cimpression ring on the pex. It still seem to crush the pipe. Does any one know of any written documentation relating to whether PEX requires a sleeve in the pipe .

Thanks
Tom Ricci
BT Builders Inc