I've been keeping them full. I have however, started and or used each saw once throughout the warm months. 91 octane (ethonalfree) + opti2
I also keep the same mix in my ice auger. Always full, never started throughout the summer. That thing always starts second pull when needed in winter. I've had it 8 years w/ all original lines and they feel and look brand new.
Side note: bro-in-law dropped off his MS290 because it wouldn't start for him after taking it for a swim with him. Apparently it was too hot too. haha.
He had it the back of his jeep for a couple days before dropping it off, then I let it sit for a couple days in the cool garage (20 deg. less than ambient).
When i cracked the fuel cap, it was like a geyser of raw fuel pushing out. I'd say he had a little tank pressure built up. Vent was plugged with water and with 100 plus degree days, I'm surprised something didn't break (fuel line, cap, vent, vent line etc.)