Hi,
Whether if having actinic is really crucial to corals does depends on a few other factos, 1) number of white lights avail, 2) number of total watt, 3) exposure timing, 4) Type of lights (FL/PL/T5/MH).
For Ivanlaw case, I'm assuming the proportion of actinic to his white light are nt more than 1:2. Thus having the actinic on for the same period as the white light will definitely helps the corals.
However, if another reefer already has 400watt of MH for a 3ft tank. A single tube of 30watt PL actinic light will not really cause any differences.
All these are my personal opinion. Anyone in this field may want to jump in to comment whether my assumptions are right or wrong (I'll like to know for sure also).
Cheers
JC