Well, there was a case a few months ago, mentioned here and on other boards, where someone managed to come down with serotonin syndrome after doing 4-AcO-DMT three days in a row while on a SSRI. The circumstances show that it takes quite a bit to cause such an interaction, but it's still a possibility with frequent and/or high dosing.
By most accounts, psilocybin typically isn't affected by SSRIs as badly as LSD. I have a few friends on SSRIs who still require a lower dose of shrooms than I do for the same effect, and one who can still trip but now requires 5g to have a worthwhile experience. Individual brain chemistry still factors into how the interaction works, so there's no way to tell till you dose.