hi there,
looks like you have some nice plants there! iam currently growing using soil and always have. it looks to me from the picture that it could be heat stress as jerry pointed out, but i would also definatly do a ph test on your soil and your feed mix as it may be a little acidic which causes chemical burning inside the plants leaves which appears as spots or blobs on the leaves showing in yellow/browny colour.

also are u flowering the plants just now? if you are just growing the plant in vegetive state then u should have your light on 18hrs and off for 6hrs per day. when the plants are the size u wish them to be then change the light timing to 12hrs on and 12hrs off as this light timing is what tells the plants to go ahead and start making some ganja lol. as for temperature and this is very important your growroom should be kept between 70f - 90f or 24oc-30oc it is very important to keep the temperature withing these zones for maximum growth and yeild.

as for spraying the leaves in my personal opinion it doesnt really matter when u spray them but personally i do it when the light is on mostly so the excess water has evaporated in the room before the light goes off again as this helps keep damp and mould away from the plants/growroom in general.

by the way have a look here for light timing info
Timing is Everything

anyways hope this helps, cheers rossco