filter then modulation or modultion then filter ?

Hi,

       I was wondering if it matters to apply a filter ( root raised
cosine, for pulse shaping ) on IQ streams and THEN modulating ( i (t) *
cos(wt) + q (t) * sin (wt) ) or to first modulate and THEN applying the
pulse shaping filter on the modulated signal ?

      thanks in advance, mel

You'd typically first apply the raised cosine pulse shaping and then
modulate. This allows you to use baseband filtering rather than the rf
filtering required if you decide to filter after modulation.