I totally agree with what jacqy79 said.
Some folks prefer to spike 7th 8th 9th & 10th.
However, spiking 7 and 9 will get you the same results. You simply tune up or down if need be from the spiked position.
I own 2 banjos, one of which is spiked at 7 and 9, the other at 7, 8, 9, and 10. It’s basically whatever you prefer.
As to your question about spiking the 12th…No, you certainly don’t need to do that. I’ve personally never seen or heard of ‘anyone’ doing that.