Read on the nikwax site.
First be clear that your clothes still should have a waterproof layer. That's the goretex, or goretex waterproof-breathable equivalent which is a layer under the outside fabric. What these products do is only for the very outside layer of fabric when it is no longer water-shedding (the dwr layer).
Techwash won't not ruin the waterproofing nor the DWR layer. It just cleans away oil and grime that causes water to lose surface tension and soak into the fabric easier rather than bead up. Perhaps though, any wash cycle does induce some wear and tear through mechanical friction and water, but it's not specifically from the techwash.
If you use wash-in to add extra DWR when you fabric already is well DWR-ed, then you are wasting money.
So do the Techwash. After the cycle is done, you should be able to plainly see if water is still soaking in anywhere. Or if you are not sure, hang it up to let it complete drying, then stick your item under the faucet to test the DWR and see if it gets wet, or if it sheds the water.
Think of it like wax on a car (wax=any modern car protectant product, even if it's not wax).
If a car is well-waxed, water beads up. If there's a good coat of dirt on top of your wax, the water pools and soaks into the dirt.
If you just wash away the dirt, but the wax remains, the water will beads up again (techwash)
If all the wax is gone, and it's just plain paint or clearcoat; then the water pools up. Get your car waxed (wash-in)
In all cases, your car is still waterproof.
I saw that nikwax has sprayon product. The spray on products could be much more financially efficient.
This site also suggests you can use 303 High tech fabric guard also, which gives you a lot of bang for your buck. It's often found in car/marine detail supply shops. I happen to have a bottle for other purposes, but have not tried it on my ski apparel