I get almost two full days of use from one battery charge of my SE. At the end of Day One I'm usually at 60-65% battery. I don't use it for sleep tracking, so I power down and let the AW rest til morning. I believe that we should occasionally restart our devices like PCs, smart watches, iPhones, etc. so this is my every-other-day shutdown of the AW.
At the end of Day Two my battery is around 15% so I just put in on the Nomad base station and charge it overnight.
I think your answer depends how long you'll be going without the watch. If it's for an hour or two you might just place it in Airplane Mode. If it's overnight or longer you can just power all the way down.