Under-torquing an SMA (or any connector) will extend its life but compromise repeatability. For quick-and-dirty at HF finger-tight is fine, Above UHF one must be more careful and so on up into the microwave bands or for exacting measurements.
One contributor to long life is keeping the connectors clean. Fine-tipped foam swabs, alcohol and canned/compressed air work best.
Here is the Keysight "bible" on microwave connector care:
Best regards, Don
Donald S Brant Jr