Yes - you can switch accounts on one device many many many times. Once you have associated that account with that device, it uses one of your 10 slots for the next 90 days. After then, that device doesn't count as a "new device" for your limited slots.
The text "You may switch an Associated Device to a different Account only once every 90 days" appears twice in the current iTunes, Mac App, App Store and iBookstore T&C under two subsections:
- AUTOMATIC DELIVERY AND DOWNLOADING PREVIOUS PURCHASES - under the iTunes store
- AUTOMATIC DELIVERY AND DOWNLOADING PREVIOUS PURCHASES - under the Mac App, App Store and iBookstore
So - if you take one of the many accounts that can exist on one device - only one account gets the "automatic" pushing of content purchases prior or elsewhere until the 90 day timeout expires.
Other than that one restriction, think more about your account rather than the specific device. Accounts are intended for one person, so when you create it, you can't go and use it on 11 devices after the tenth new device your account hits, you need to wait 90 days from the first device to account pairing.
It doesn't matter what other accounts are on a specific device - its about your ID being used in the app store on multiple devices.
For questions like this here are some nice references: