You can label either of the primers for T-RFLP. If you wish to validate or get absolute confirmation on number of T-RFs generated, you can use both labelled primers but with different dyes. You should ideally get same number of fragments using fwd as well as reverse primer.

Hi, as example, a fluorescent labeled forward primer is used for C. difficile ribotyping.

you will need to think about the  fragment size and where the rflp is within your fragment. Label the primer that gives the larger cut fragment size as very short fragments may not run well, may diffuse more and if very short may even be obscured by the signal from the dye labelled unincorporated primer