It varies based on the SARM (and the other items people lump in with SARMs).
LGD causes complete shutdown at even small amounts. PCT is vital here.
RAD140, OSTA, and S4 cause minor shutdown. I would recommend a PCT because I am paranoid. I have low t and am married to the needle and would rather see you spend a few bucks on a PCT than possibly break yourself.
MK677, GW50, AICAR, SR9009, and YK11 are not technically SARMs (they are lumped in with SARMs since SARMs sellers also sell these) and are not suppressive at all.
The test studies done on humans for LGD said everyone who took the drug fully recovered their test production without any PCT - but I say why risk it? It is better to be safe than sorry, especially when safe is easy and sorry is miserable.