Is it OK, no!

The problem is a mix of air and water can, over time, create all manner of nastiness. Acid to dissolve metal, sediments, maybe even fungus.

Two ways recommended:

• Empty the boiler and store it absolutely dry. (Hence blow-down valves)
• Completely fill the boiler with well boiled pure water (to remove air), ideally with an inhibitor.

For long-term storage, also necessary to completely seal the boiler to stop air getting in. Full-size boiler maintenance gets extreme – I've read of power-station boilers being mothballed by draining down, flushing, applying a chemical treatment, and carefully drying out, before filling with Argon at 5 psi and then sealing!

Finding really pure water for long-term storage may be problematic too. Tap-water contains dissolved minerals in hard-water areas. Soft-water is better, but tap-water always contains Chlorine and other chemicals for public health reasons. Boiling it for several minutes or more helps considerably. Rainwater is polluted, even worse if it's collected off a roof. Distilled water is expensive leaving de-ionized water as sold for topping up car-batteries perhaps the cleanest water readily available at reasonable cost.

Dave

As water is evaporated it concentrates whatever impurities are in it so the purpose of a blowdown valve is to get rid of this concentrated solution and gunk that settles near the bottom of the boiler. These impurities also disrupt evaporation during running so you can often find a problem with the boiler priming (carrying water over in the steam) can be cured by a blowdown. A friend's commercial feed mill heats the ground ingredients with steam from a boiler the size of a sofa. It automatically blows down every 30 minutes.
With a copper boiler leaving it standing between monthly runs should not be a problem, longer term fill right up is going to be easier than drying out.
Rainwater filtered of course is a good source and the best is probably from your workshop dehumidifier but again filter it to remove dead flies and keep an eye out for the dehumidifier's condenser starting to corrode.