Run it ll the time and on fresh, use recurculate when you need it. Auto resurculate will prevent washer smells.
All this keeps the system fresh, odour free and it also keeps all the seals lubed. There is trace oils in the system which keep the seals etc. lubed but this doesn't happen when its switched off. Also, if the system isn't used it will start to smell, yo'll notice when you switch back on then it will start to smell all the time.
I keep mine on 20 all year round, have done on every car i've owned and I have never had an AC or climte issue. Regular re-gas/service is the only time I have had any work done to the systems, I think this is mainly down to have the system perminently on.