For bearded iris, trim only dead or injured leaves and cut off the flower stalks after they finish blooming. The plant stores up food for next year's growth in the rhizome. Cutting off the leaves may hamper food production/storage which may affect flowering the next year.
If you need to transplant iris, do it in August. Then it would be best to trim the leaves back to about six inches, just so they will not tip over the newly-planted rhysome.