Why not keep them on 24/0?

Unless you are concerned about the electric bill, you might as well keep them under 24 hours light, especially if they are mother plants and are going to be staying in veg.

Cannabis does not need a break in their photoperiod during vegetative growth, meaning they do not need any periods of darkness to perform photosynthesis efficiently. More light = More growth, and with the 18/6 light cycle you are losing approx 20-25% of potential photosynthesis and growth.

Again, if electric bill is a problem, or temps get too high, then it is understandable to have them on 18/6 . . . otherwise go for the 24/0, they will grow heartier and revive more quickly after taking clones.

Best luck,
JPG