Not having any dihedral is bad. Dihedral is used to stabilize your plane. Without it, a slight upset in flight, perhaps just from turning, will not be recovered from and could be accelerated. To describe this, I will steal a paragraph from the pdf linked to below:
"To describe how dihedral provides lateral stability,
try to visualize the airplane’s wing not moving but having a flow of air pass over it. If the wing is held so that the air passes at right angles to the LE, it is the same as it would be when the
airplane is flying straight. However, if
the wing is rotated to the side so that
the air passes at an angle to the LE,
it is the same as the airplane
flying in a circle. When the wing
has dihedral and is angled to the
wind, there will be more wind under
the side of the wing that would correspond
to the inside of a circle and also more
wind above the side of the wing that would
correspond to the outside of the turn. This
difference counteracts the tendency of the
airplane to go into a spiral dive in the same
direction as it was circling."
You don't necessarily need one dihedral point in the center of the wing, however. You can have two dihedral points with a horizontal center, like the picture in the pdf. Or use tip plates, which basically are an extreme form of two dihedral points, with the tips going straight up. Some have also had success with little or no dehedral on the wing, but some form of dihedral on the horizontal stabilizer. This will be less stable than dihedral on the wing, but it may be stable enough for a wind-current-controlled gym.
pdf:
http://soinc.org/sites/default/files/up ... iad5.0.pdf