Singles slant happens in weaving just as it does in knitting. (If you let go of it, does it immediately start to unspin itself, play back on itself, or get into a tangled mess? Then your singles are still “active.”) If your singles yarn is still “active” when you weave with it, your work may slant to some degree.
Regular weaving, you might think of as looking like this, since I can’t find any good images online:
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With active singles slant, it might look more like:
…./ / / / / / / / / /
.../ / / / / / / / / /
../ / / / / / / / / /
./ / / / / / / / / /
/ / / / / / / / / /
(The dots are there to hold the place since spaces don’t stay. Basically, a piece you might want to stay rectangular or square could look more like a trapezoid.)
If your yarn is balanced (this is what plying does, or you can make a balanced singles by steaming it or by not putting very much spin in the yarn), singles slant shouldn’t be a problem. However, a plied yarn has the advantage of strength and durability, which might be a factor when making yarn for a rug. Is it for a high traffic area, or is it something you plan to hang on the wall, for example? That might tell you how well you want your yarn (and rug) to be able to withstand wear. Also, if it’s going to get dirty and need washing, a less-spun or singles yarn will probably shed more of its substance, or become “fuzzy” as the ends of each individual fiber come loose.
One more thing you might want to think about (although you didn’t bring it up) is the suitability of an undercoat for rug production. I bet the undercoat is very fine and soft, and would be excellent for things that are delicate or next to the skin, like a scarf or sweater – things that typically don’t take a lot of hard wear. Wool that is used for rugs typically comes from not-so-fine sources, like an outer coat or a “hairy” (as opposed to “woolly”) sheep, like a Navajo-Churro fleece, that is strong and scratchy but can take a lot of wear and abuse.