Our slides are open only if we're using it. It's JMHO but I see no good reason to leave them out, and several good to leave them in. But to each their own.
2000 Ford F350 XLT 7.3L PSD CC 4x4 OffRoad SRW Long Bed
2008 Jayco Eagle 314BHDS (Momma Eagle)
Equalizer Hitch System (1400/14000lbs)
Prodigy Brake Controller
Curt XD Class V Receiver Hitch (1500/15000 lb)
If not using it for a long time, they should be in. Put them out when using the TT. It is actually good for the slides to be retracted or extended occasionally.
This is in my user manual. IIRC it states to exercise the slide every couple of days or once a week if you are using it.
When not in in use, or not being aired out/prepped for next trip I keep the slide in. Ours can sit in our driveway during the summer when we are not camping and it just makes it easier to work around it when the slide is in
2011 Passport 290BH Reese Straight Line Hitch Tekonsha P3
2009 Silverado 1500WT ext cab 4x4 5.3L V8 3.73 tow package
I understand the conveniences of leaving them out when you come back to the camper. No question there.
But, there is always a but... If you have ever dealt with rotted slide floor or seen slides rotted that are on seasonal sites with the slide left out, the in-convenience and cost of dealing with the repair changes ones mind real quick.
Retract the slide in when you leave for weekend, or extended periods. The slide seals are breaking down in the sun all that much more, If you have a topper, that is getting sun baked on the entire top, your odds for water getting in have gone up by orders of magnitude. Even if the slide is old enough to be a manual crank out, pull it back in. That 2 to 3 minutes of in-convenience is small in the big picture of things.
See my slide floor rot saga and this was from the prior owner or worse sitting on the dealers lot. The camper was 2 years old when I bought it and about 1 year on the dealers lot. And if your slide has the black membrane liner, make sure it does not have these exposed holes. Many brands still today are built just like this.