It probably does no harm to get the grain right, but my experience is that papering the fins certainly does make them a lot stronger. That's with basic copier paper and white glue, though - other coverings may be less effective. I have one rocket whose fins are the wrong way round (*) and which is as strong as any normal rocket now its fins are papered. Another one has the fins correctly aligned for grain but broke them anyway on its first two flights. It has two large horizontal fins with smaller vertical fins on the tips, and those vertical fins kept breaking. A similar size rocket with long, swept back (unpapered) fins using the same size parachute has survived many landings without damage, so it's not the parachute that's the problem. Anyway, I papered all this rocket's fins and they survived the next flight without damage.
(*) Intentionally. It was a kit with long thin fins that weren't likely to survive transport, let alone flight and landing, so I turned them 90 degrees to make the rocket easier to pack.
