In the early game it makes far more sense to keep your assets as catnip until you need to spend them (or your barn fills up).
Your strategy only works if you have enough farms to cover
your base needs (that's a lot of fields). With 10 cats you need to be producing 40 catnip/second baseline.
5 farmers and 5 woodcutters will only work if you're producing at least 15 catnip as a baseline. On an average winter (-75% from fields) you're screwed.
On the other hand if you leave 10 farmers running, you'll generate at least +10catnip/s even if you have no fields. It's simply not worth it for the extra half wood.
In the beginning, I kept on buying fields. But even if not, one or two farmers can cover a bunch of kittens, and then marginal woodcutters are better. Until later, in which calculation is pointless unless you really care.
100 catnip->1 wood. Farmer=5 catnip/sec=100 catnip/20 seconds=1 wood/20 seconds Woodcutter=.07 wood/sec=1.4 wood/20 seconds.
Multiplying both by five changes nothing.