When should I evolve Roselia into Roserade?

Level 50. Roselia doesn't learn any moves after Level 50, and any extra moves learned by Roserade must be learnt via the Move Relearner.

Is Roselia evolving worth it?

Roselia's evolution, Roserade, is the best grass-type Pokémon in the game for raiding, so it's definitely worth picking a few up while its spawn rate is increased. As with other Community Days, Roselia's Shiny rate will also be increased, making it easy to find one.