Well,
96 inches/12 inches per foot =8 feet
108 inches/12 inches per foot =9 feet

I thought it was pretty straightforward (as the math would indicate) but ran into issues with the crown moulding that I failed to factor in. So my 108" curtains for 9ft ceilings needed to be hemmed. I have found the Ikea curtain length of 98" works okay with the rods hung about mid-way between the crown and window trim. Not perfect but I can avoid the extra expense of hemming.

Good luck!

To follow what Annie is saying... basically it all depends on your windows and personal tastes. I do not prefer the curtain rod to be almost at the ceiling, but that's my opinion. My ceilings are 8ft and I have 84" curtains. The rods are hung about 2-4" (depending on whether I was using grommet vs curtains on rings) above the window casing and about 6in wide on each side. My curtains come to just above my shoe molding, which is too short for many here, but I didn't want them touching the floor.

IMO, when the curtain rod is all the way at the ceiling, it loses some of it's relationship with the window. Of course, there are examples I've seen that I like, but I don't think the curtain rod at the ceiling is right for every window.

You really should hang your curtain rod first, then measure from the rod to the floor. A lot would depend on whether you are using rod pocket, or rings and clips. The rod pocket ones would need to be a little longer. With rings and clips you would need to hang your rod at least a few inches above the window or you will have sun shining in above the curtains. I have furry dogs and don't like the fuzzy curtain hems so I hem mine just above floor level.

My goal would be to mount the curtain rod where I want it and end up with curtains that are no more than 1/2 inch off the floor without touching the floor. I would buy whichever length is long enough to do this and then hem them accordingly.

I'm a retired seamstress. I always had my clients hang their rods first.
For rod pocket, back tabs, or tab tops, measure from top of rod down to where you want the finished hem to end.
If you are using clip or sew on rings place one on the rod and measure from where the top of the panel will start, down to where you want the finished hem to end.
This will also help you determine what length panels to purchase.

I would like to say thank you so much to everyone that has responded! All of your responses have been most helpful and given me things to consider that I hadn't thought of.

I have a small living room and I want ceiling to floor grommet curtains.. Should I hang hem so that they 'break' on the floor or just meet the floor?? Thanks!!

If you're going to open and close them, then just off the floor. If stationary, it can be either (a preference thing) however, personally I prefer grommets just touching or 1/4" off the floor.