I don't know if there's a "best" time. There definitely would be a worst time, in the fall when the tender new growth stimulated would be hit by severe frosts. Leave enough time for the new growth stimulated to harden off a little before frosts hit.

Many gardenias show buds as the nights lengthen and cool off in fall, so I would think now would be a good time.

I find it useful to follow the 1/3 rule-- cut back 1/3 of the branches, and cut back by no more than 1/3. When you get resprouting on the cut branches, you can cut another 1/3 of the branches if it's not too late in the year.

This will thicken the plant over a period of time without completely denuding it of life-sustaining foliage.

Vgkg,
Gardenias usually set their buds shortly after blooming.
When did your gardenia bloom?
Look closely at the bush to see if you can see the new buds set.
If you can't see any yet, there is a small chance that it hasn't set them yet.
Personally, I would wait til next year after it blooms, but if you really want to trim them, do it if there are no new buds set yet as soon as possible.
They are usually definately set on most species by August 1 in your zone.