Not a silly question. Not a simple answer.
I had a Vanguard account and I was invested in admiral shares (which have a higher minimum and lower expense ratio) of a particular bond fund in my Roth IRA. I exchanged into other funds, leaving the balance below the required $10,000 minimum. I received a notice to shape up or ship out. I don't remember which I did, but I took care of it.
Conversely, I opened a Vanguard money market fund ($3,000 minimum) and promptly defunded it to $1. It exists solely to collect quarterly dividends. Apparently, they find this behavior perfectly acceptable.
Long story short, YMMV.