If you trim them by a third or less, you will probably get more blooms this summer. Lavender does get woody after awhile, despite trimming. What I do is layer them to get new plants, thus renewing my lavender patch. To layer, pin a branch to the ground and cover with soil. Keep it watered, and by summer's end (or wait till spring) you should have roots where the branch was pinned down and you can then cut it from the mother plant. A very easy way to get lots of new lavender plants.
HTH
Christine

It depends on the species and cultivar. L angustifolia cultivars are generally more likely to throw a small second bloom if lightly pruned just after blooming. Some cultivars will bloom a second time without any pruning. L. xintermedia are much less likely to bloom a second time ever. However, I did have a lady claim that her L xintermedia 'Seal' bloomed for her three times in one season! Very unusual indeed. Two cultivars, introduced to the trade by Tom DeBaggio (my dad) are fairly reliable re-bloomers 'Irene Doyle' (AKA Two Seasons) and 'Tucker's Early Purple'. In some areas of the country, L. stoechas re-bloom constantly, although it usually only blooms once in my area, late May/early June.
I recommend to people that experience hard winters ie have many days below freezing that they prune their lavender plants one-third to one-half of the plant, not including the length of flower stalks, in early spring. In Northern VA that means in late March. Hard pruning in the fall can trigger new growth that will be damaged by an early freeze. Tidying up of spent flowers and light pruning can be done in fall if desired with little need for concern.

This, early, fairly aggressive pruning will prevent the plants from open and woody. The plants will still get thick woody trunks but the plant will look much nicer. I hope you have better luck growing lavender in a pot than I do. They usually don't last more than a couple of years for me in pots.

F. DeBaggio