Like all lavenders, they do not regenerate from old wood, which means they should be pruned back 2 or 3 times a year, after a main flowering, to keep them bushy. Clipping over involves removing all the spent flower stems, plus about an inch, but no more, of leafy growth, unless there isn't an inch of new, soft growth, in which case, take less. Lavender also becomes leggy and woody with age, so it's usual to replace plants roughly every five years.
I don't know what your climate's like (I think Seattle is very, very wet, but could be wrong) but I think you're in Fall now and if your winters are usually hard, then a light clipping now is about all you should do, particularly with French lavender, which is slightly less hardy than other varieties such as Dutch or English lavender. Clipping a lot off might encourage new growth which will not have time to harden off properly before cold temperatures arrive, IF you get those around November time.
UPDATE: Old and new wood is a bit of a misnomer - the new growth isn't really woody, its grey and soft or softish. Any woody parts are old really. I'm trying to think when Thanskgiving is - around 21st December, maybe? I don't know when Autumn officially starts where you are - in the UK, it's September, when the days are noticeably shortening and nights are cooler, but we wouldn't expect frost before November most years.I would, now in the UK, clip over the bushes very lightly; since you can't cut into woody parts anyway, you can do the same if your weather patterns are similar and definitely if your Autumn is later.