My priority list of strategies to use for IB:
1. Use before a predictable hard hitting ability
2. Use when healers is forced to move to avoid damage (or is disabled/distracted)
3. Use Off CD

Note:
#1 is fairly obvious for when these situations exist. For the most part, if a guide states to blow cooldowns for a certain mechanic, IB should always be included.
#2 is lenient but helpful for fights like Ultima/Mog/Turn 1 (low stacks)/Turn 2 (Firestream/Ballasts) where the boss will continue it's basic attack while other mechanics take place that can steal a healers attention. The logic is, if there is a situation where a healer may be forced to move, and point #1 is not in effect, then use IB during that situation. I personally will (attempt to)keep an eye on a healers cast bars and if there is ever a gap within their casts, then I pop it for more support.
#3 I'm lazy and/or the content is trivial. I also use this when damage is constant and extreme where any (and all) mitigation will help survive.