Good question
@sewingj. I also don't deal with striped knits very often. When I have, I've also cut stripes in a single layer and concentrated on getting them horizontal on the garment piece, matching at seams if required. That might mean that my pattern piece ends up skewed, in relation to the normal grain line direction and not parallel as we'd normally aim for.
That's okay, we're allowed to break the rules from time to time...
My striped garments have still remained 'square' with no side seam twisting (something I hate), even though 'off-grain' by traditional techniques. And since the stripes are then the dominant feature, it's best they look correct and horizontal, rather than worrying about whether the courses (the vertical lines in the knits) are exactly vertical.
More curious (or possibly incredibly boring - read at your own risk) technical snippets about knitted stripes:
- Knitting coloured stripes in the fabric, using coloured yarns, is more risky for the manufacturer - what if the colours they've knitted in don't sell? Stripes may instead be created using two different fibres and then dyeing the fibres with their respective dyes at the last minute, so the manufacturer can respond faster to what is selling in store. If you buy a poly/cotton knit, one stripe colour is likely to be poly, the other cotton, so that different colourways are created by just dyeing the cotton to the required colour - the polyester won't be affected by cotton dyes and will stay white.
-Striped knit fabrics have 'skew' built into the stripes (as do all circular knits, but it's more obvious in striped fabric!) - if you've ever hand-knitted in the round yourself, you'll know that you are actually knitting a spiral and our one thread ends up on top of the round before it. In commercial knits, there are dozens of yarns being knitted around at the same time, so one yarn ends up being separated from its previous round by several centimetres.