Your cockatiel should not need to have his tail feathers trimmed, contrarily to popular belief. Some cockatiels may thrash their tail feathers in the course of their normal activities and you may feel better about your bird’s appearance if you trim the scruffy-looking feathers off.
If the tail feathers are getting damaged often or look ratty all the time, it could be that your cockatiel’s cage is too small to allow him to move around easily and comfortably. Make sure the cockatiel’s cage is spacious enough to extend his wings fully and not have her crest touch the cage ceiling or the tip of the tail feathers touch the floor.
Hope this short bit of advice serves you well.