Yes, I always preflashed direct positive paper. I've found for my process that the Harman Direct Positive FB paper requires about half the preflashed exposure that I would normally give grade 2 paper negatives.
Conversely, the Efke RC direct positive paper, that came out a year or two ago, required much longer preflashed times than I would normally give paper negatives, almost 4x the value.
So obviously I had to figure out these preflashed times for myself.
How I determine the required amount of preflashed time is to set up the darkroom in the daytime, pour up the standard chemicals, set up a scene outdoors with which to photograph, and do a series of exposures with varying preflashed times. After development, the prints are examined when fully dry and the optimal print reveals the best preflashed time. Of course, you do this only after already having figured out, via a similar calibration test, what your working Exposure Index will be for the paper. I rate the Harman Direct Positive FB paper at ISO 1.6; others report slightly different values, which is okay. The important thing is being able to do your own testing, to arrive at what exposure times work best for you.