Its not unusual for a large piece of wood to have internal stresses inside itself that cancel each other out. When you remove part of the piece you have two pieces with unbalanced stresses. One or both pieces may take on a curve. You can live with some curve in your strips, in fact having a few strips with some sweep to them may be desirable. What is important is that the cross section dimension, usually 1/4 inch by 3/4 inch, remains constant.

Unfortunantly, I don't know the best way to eliminate this problem, but I'll bet that there are some woodworkers here that do.