instead of having that decision made, I try to attract a more possitive outcome, and be proactive. I don;t have to make that decision if my horse doesn't colic. I also live in the barn, so I see the very very very slightest and earliest signs of colic and deal with it then. My horse has never colicked. But there are a few horses here that tend to.
The risk of impaction colic this time of year is pretty high, depending on where you live. The grass gets dry and doesn't contain a lot of moisture. First and foremost, if your horses are on grass right now make sure they have access to PLENTY of water. If the pasture is big, several water buckets is a good idea. Also adding beet pulp soaked and soaking their grain will help increase their water intake.
Also adding probiotics or live cell yeast to their diets just to aid in digestion, is a good idea. IMO.
Good luck :)