Personally, I wouldn't leave ANY mech with a bottom firing button left standing up without it being locked - even if it does look stable. The chance of it pressing down and firing without any safety
devices to cut it off after X amount of time really worries me!
Though some may disagree, I think with unregulated devices we should all do our best to reduce the risk of a failure
as much as possible. It makes me nervous to think some are leaving their mods standing up for a long period of time, maybe unattended, without it being locked.
Of course, if you're going to be with the mod at all times while it's not locked then it's a bit different (though I still think it's a good idea to get into the habit of locking the switch). I'd highly recommend a mod with the firing button recessed - some of the mods on the market with a recessed firing button are just BEAUTIFUL
With magnets on my Nemesis it stands up without wobbling but some of the heavier rebuildables tend to make it a lot more unstable.