Rinsing in freshwater is an absolute must for the swing arm hydrometers. However, try testing 3 times in a half hour and tell me what readings you get. I found they are not consistent and therefore unreliable.
I calibrate my refractometer once every few months, or if I think a reading may be off. It takes under a minute to check it and I know it's accurate. Stable SG
is one of the most important parameters to maintain. For the few extra dollars on a device that lasts seemingly forever is well worth it.