Host side question about Clap Gui create/destroy

[mutools]

When should the host call gui create()/destroy(): On instanciating the plugin (after plugin.init() but before plugin.activate()), or each time the editor window is opened/closed?
I tried the first but it turned out that some plugins don't like that.
So i'm now calling gui create()/destroy() each time the editor is opened/closed, which seems to work, but isn't that a waste of cpu resources? Why would the gui have to be created/destroyed upon each editor open/close?
If it's obligatory for a host to call gui create()/destroy() on each editor open/close can you please specify this in the docs.
Thx.