Just wanted to add my two cents here regarding the effects of /assist and resets.

All my instance macros for all my DPS toons follow this structure:

/assist Arvault (obviously not used on Arvault himself)
/use 13
/use 14
/use (other non-GCD ability here)
/castsequence reset=combat/target/# spell1, spell2, spell3, ...

I've encountered no noticeable impact on casting for any of my toons that you see in my signature. If you are, I might suspect something else may be interfering. Addons aren't likely to mess with macros, so I'd take a look at what else you may have in there. For casting classes, I don't see much value in having /startattack, since that just makes them try to swing with their melee weapon when in range, so I'd try taking that out.

As soon as I stop slacking off and devote some time to it, I plan to move from /castsequence, to the /click methods mentioned on other cast rotation threads on this forum. Not that there is anything wrong with my current macros, but /click will help improve DPS just by reducing the effect of latency.