I have not used simcoe for bittering at all, I only use it as a flavor/aroma hop.
However, I really like magnum for my “single hop charge” beers. I find it has a more rounded bitterness that isn’t quite so in your face. If I remember right, simcoe has a bit harsher bitterness.
Why would you waste Simcoe as a bittering charge with all of those wonderful flavors and aromas it provides. That hop is too precious and hard to get to waste it as a bittering charge! You are going to boil most of those aromatics off, anyways. Substitute it for another high alpha hop like Magnum, Chinook, CTZ, Summit, or Apollo. Save the Simcoe for late in the boil. That’s my $0.02.
Go for it. If that’s what you’ve got on hand, there’s no sense in not using it. Columbus is one of my absolute favorite aroma hops, but I still bitter with it all the time. There are so many awesome aroma hops out there, that if you can’t get Simcoe at some point, it’s just an excuse to experiment with something new.