Paul,

The need for strength is not that simple.
Glass is a lot stiffer than cedar, so the ability to resist bending comes primarily from the glass - more load gets picked up there.
How much is necessary?
Well you could make the cedar thick enough to do just as well in bending on top and bottom of the boat even without the glass.
But in bending, you need "shear" strength on the sides. The best direction for shear is layers at a +45 and -45 degree angle to the length of the boat. It doesn't matter if you are talking glass or cedar.
How much is necessary?
To know exactly is a difficult question.
With enough thickness of cedar, even straignt fore and aft can take the loads. For a certain thickness it might be good enough or it might rip the wood up along the grain.
With glass even in the normal direction (warp parallel to the length) you have 50% of the fibers preventing the grain splitting.
With the glass/ cedar/ glass sandwich in a normal direction it seems to work at a reasonable weight.
If you want to get every last oz out you could do a lot of analysis, lots of testing and you probably could get a weight reduction with lots of additional construction effort.

But this is not the only real world situation you have to survive.
What about launching a boat resting on a rock that is trying to punch thru the thickness of the hull? Now you need to keep the outside layer from crushing/ crimping/ breaking and the best orientation is plenty of layers at multiple angles. Unless you can get some really thin glass to orient at different angles you probably will be adding significant weight for improved resistance. The wood will still tend to split along the grain, so you need an additional direction (a new layer). The inside glass will have a very high load in all directions so you need the same multi axis orientation.
All that is theoretical, and depends upon finding or making the exact right sized glass and cedar layers to not over build the boat.
What you get current typical construction with cedar strips and plain weave glass running fore and aft (warp) seems to work well over all. It has a nice balance of properties.

How much are you willing to spend for improvement - both money and time?

Please remember also that local fasteners/ hard spots / penetrations probably need something else to be done to be "best".

Note that the Gougeons (West System) have done lots of testing scientifically and suggest putting the inside ply at a 45 degree angle.
Personally I will just do that and say "good enough".

But if you do all the work, please publish so I can tag along.