I open with IM on bosses for the following reasons:
1. It allows you to use rail shot and get the extra damage due to burning target.
2. It won't proc PPA Railshot
3. It gets your IM dot rolling.
1: Flame burst does the same. Moot point.
2: This is a valid point.
3: I'd contend that I'd rather miss 2 ticks of IM DoT (376 per tick on my ship dummy) than miss 2 -3 (3 if you go IM-RS-RP, 2 if you go IM-RS-FB) of the Cylinder DoT (560 on my ship dummy)
4: I still pop off IM after my second rail shot, so it's going during my cooldowns.
Mind you, I'm still just playing around with this, and I haven't been doing Pyro all that long, so I'm not saying that I'm for sure right on this.