Is it necessary?? No...unless you want a textured wall instead of a smooth wall. Any type of texture finish in a kitchen is a bad idea anyway...tends to collect grease from cooking and dust just sticks to it. Next to impossible to keep clean and always a pain to paint. Just clean off all residual glue after wallpaper removal, fix any imperfections/cracks with spackle compound, sand smooth to blend, prime, and paint with quality washable paint.