If we're talking of flex as being 'bend under sideways loads', then the higher the tension would equal less flex.
We're not concerned with spokes bending sideways, since that only happens when you have a stick through the spokes and your problems are about to get much bigger. Rims move to one side under load, and it is spoke stiffness (resistance to being stretched, not resistance to being bent) which opposes this. Within the elastic strain region, the resistance of a spoke to being stretched more is pretty much the same regardless of how much preload it is carrying.
We're not concerned with spokes bending sideways, since that only happens when you have a stick through the spokes and your problems are about to get much bigger. Rims move to one side under load, and it is spoke stiffness (resistance to being stretched, not resistance to being bent) which opposes this. Within the elastic strain region, the resistance of a spoke to being stretched more is pretty much the same regardless of how much preload it is carrying.