You should use PBS (1X) to dissolve MTT reagent. Make 5mg/ml MTT solution.
MTT powder dissolves slowly in the buffer. Vigorous vortexing is needed to dissolve the powder completely. Sterilize the MTT solution by filtration. MTT preparation in media is usually not recommended.
It is more advisable to prepare MTT solutions (both stock and working) in sterile PBS. Your media may get contaminated at any stage resulting in loss of dye solution. Prepare and dilute your MTT in sterile PBS and don't forget to filter it before use. This would remove any undissolved stain or debris.