While it's not required for an external strongback to be perfectly straight and level, it sure makes things easier if it is.
'Rigid' and 'stable' are the things I look for in a strongback. An internal strongback has to be small, but an external strongback doesn't, so 2 x 8 dry construction-grade lumber in a 'T' is what I've used for a strongback.
Spray paint over a string to establish a centerline on the strongback, set the molds square to that line.
I agree with having a 'flat' top on a strongback - it's easier to set the molds in relation to the top of the strongback, avoiding the necessity to shim the mold supports. The important thing is that the waterlines and centerlines on the molds are aligned perfectly.
By level, do you mean that 'uniform flat quality', or level as in level/plumb (i.e. parallel with the surface of water in a pan under the strongback)?
I don't think having a perfectly level strongback makes things much easier when building a kayak.
When building a bigger (immovable) boat, it makes complete sense- the WL on my sailboat build was level; and there's no strongback when building larger boats.
My garage floor isn't level, so when I move the strongback around it changes level a bit anyway.