Healing is one of the few areas where macros can be acceptable, as long as you don't overuse them or rely on too many. Mouseover macros for ST heals can help some players because you never have to tab or switch target. If you fumble even a little doing this then the loss is much larger than the fraction of a second loss of ability queue, especially since ST heals are used much less often than dps abilities.

One important thing with mouseover macros is never to add any line other than <mo>. For example:

/ac Cure <mo>
/ac Cure <t>

...has a chance to heal yourself if the <mo> line fails due to pressing the button a split second early or your target isn't in range. In a dungeon you might not notice you're spam healing yourself due to panic and there's a dead tank. Where as:

/ac Cure <mo>
/ac Cure <mo>
/ac Cure <mo>

Will either always work on your mouseover target, or simply not work because you aren't in range so you notice and adjust immediately.