Must an admin theme be enabled to be used? I've created a sub-theme of the admin theme, Seven, in order to override some CSS used to display date combo inputs on a node edit form. The info file lists a new style sheet and the regions that are defined in the original Seven info file.

Everything seems to work fine if I select this sub theme as the Admin without even enabling it. I can easily override the CSS.

However, if I also enable the sub theme, blocks suddenly appear inside the admin theme whenever it's invoked*. I believe this is because the new sub theme has different (default?) settings for active blocks in the block table. (rfay has a solution for this problem here: http://randyfay.com/content/changing-theme-name-or-adding-subtheme-without-losing-block-and-theme-settings)

My question: Is it necessary to enable a theme/sub theme that will only be used as an Admin theme? If so, is there any way to avoid the block problem, or is that just the way it goes?

* Edit: A lot of blocks appear in the new admin theme when it is enabled. Below is a visualization of the blocks table. Before the custom theme is enabled, it does not appear in the block table, so there are no blocks at all. After it is it enabled, the 12 blocks from the main theme ("custom" in the table) are all placed into the content area. Blocks might end up in the Content region because regions in the enabled theme don't exist in the customized seven theme? In any case, it's just a nuisance really, and can be easily fixed using the SQL at the link above. However, if there is no downside in using an admin theme that is not enabled, the block clean up wouldn't be needed.
No, it's not needed to enable theme to use it as an admin theme. You only need to enable it if you want to alter it's settings, both the ones in "appearance" section, and blocks. Just go to http://example.org/admin/structure/block/list/seven and http://example.org/admin/structure/block/list/your-seven-subtheme and set up the blocks the way you want them, that's all.