Priming can be skipped while repainting using a flat paint (matte finish), though it is always advisable to apply paint on well prepped and primed surface. But never give it a miss when 1. Fresh painting a surface/drywall 2. Using gloss/high sheen paints 3. Painting a drastic colour change - lighter shade over a darker one and vice-versa. 4. Painting on repaired walls(patched areas) or walls in bad condition – paint flaking/peel off 5. Painting wood surfaces