The only time I currently foliar feed is when my clones are rooting, a very very mild solution.
The only time I would recommend foliar feeding is if you had a nutrient deficency, you could give the leaves a spritz.
-Always use a mild (no more than 1/4 normal strength) solution.
-For an added benefit, mix nutrients with sparkling i.e. seltzer water--the plants love the extra shot of CO2.
Make sure what you buy is just carbonated water--some brands contain sodium, which you don't want on your plants.