If you don't plan on getting an ARB type of bumper, I'd mount it on the hood like in my
pic here when it use to be there.
I got a very low SWR match of around 1.3 or so.
But now that I have the ARB, I prefer
it there.
The antenna is now gone in exchange for a
black 4' Firestik II.
The SWR match is even better on the ARB.
I have about a 1.1 across all 40 channels.
Center of the roof is best as mentioned, but may be inconvenient for some.
Check out Jim's post
here.
He really likes his, and it comes off real easily for parking in his garage.
The shop did a good job, and no leaks.
That is the main thing for a roof install is to do it right the first time if you go that route.