My general rule is:

cooldown less than 3 seconds? use reset=target/combat
cooldown less than 6 seconds? use reset=target/combat/6
cooldown more than 6 seconds? use reset=combat

I make the final decision based on other spells in the sequence

Note that I don't use clicksequences or single button spamming either and often run mixed groups - so I customise the macros based on group makeup, target type, etc.