Can tell you what I do:
Shield Lob -> CoS -> Flash -> FB -> Flash -> SB -> Flash -> RoH, then alternate RoH.
This is on big pulls though, on smaller ones you'll only need 2 Flashes. Obviously if you have a lot of mobs and a lot of AOE going on, you'll want to do more than just 3 Flashes. RB on mobs that are left over at the end of all the madness, and move on to the next pull.
I don't know if you or the OP mean using raw RoH, in which case don't do that. Flash and CoS are enough to hold AOE aggro, doing the RoH combo is for damage and single target enmity generation if needed.
Also to answer your question more directly: you want to have CoS ticking/on CD ASAP so you should use that before you actually Flash. Typically I pull more than one pack of mobs, so my rotation generally doesn't start with Shield Lob but rather CoS -> Flash. From then on you have to decide what you need more of: AOE enmity or damage on single targets. If you need more AOE enmity, you'll want to Flash more - but you don't need to Flash every single GCD. You can start your RoH combo and Flash in between the sections of it, giving you both the single target enmity/damage and AOE threat generation.
