Reading some of the other (very informative) posts I have to ask.
Where would you have connected the relays had the recommendation been to leave them connected all the time?
Personally I do not like these relays. There have been a number of posts where they have failed. And because I don't know who mfg them I wouldn't bet the farm on them being safe. I know they are made by a company who puts the name Songle on the relay but there is no historical record of their performance. A company like Allen Bradley, TE etc has industrial parts in the field by the millions. Folks know they are a quality part.
One also has to worry about what might happen if the relay failed and sent mains voltage to the Arduino and maybe the switch in your hand.
I also believe their UL mark is bogus based on missing some info required by UL.