No ... you don't need it and it would be next to impossible to line it up with your routing from the top.
In effect, your 1" deep routing from the top creates an accurate template that your routing from the flip side will ride along. As long as you are using pattern bits (where the bearing and router are precisely the same size) you can't go wrong.
Take your time and think about each stage as you do it ... you'll be surprised how straight forward it is.
Just to clarify some things from an earlier post ... when you route from the top, make sure that the shank of the bit is fully and securely locked in the collet of the router. And then make sure that the guide bearing through the base of your router by the same distance as your template so that it will ride along the edge of your template. Don't plunge the router as you go, lock it down in the correct position and eyeball it before you start routing.
Then when you're sure you've got it all lined up, start routing.
But something that may have confused you ... you're routing will be consistently 1" deep through the thickness / side of the body if you've locked your bit in properly ... okay. But depending on how closely you cut out your guitar shape, there will be a quarter inch or so that will be visible past your template when you view if from above. That's the bit that people are telling you to take gradually.
If you use a half inch bit, don't shave more than about a quarter of an inch at a time. I'm sure your router will let you do it, but you'll blunt your bits ... that's all.
Go for it ... and post some pics if you want to double check anything.