Relevant data: do you have a stem with 2-bolt clamp and removable faceplate? These can be more forgiving, but shims are still good to ensure that the curvature of the faceplate and stem interface cleanly match the bar.
I don't think the comparison to buying jeans is apt at all - these are high-tolerance measurements, and nearly all 25.4's I've seen line up with nearly all 25.4 bars, and don't line up with 26.0 bars. These surprisingly small differences matter - my Cinelli XA stem (with a 26.4mm clamp) wouldn't clamp down on a 26.0mm bar.
I'd use the shims anyway, unless it's possible that the stem was mislabeled. I bought a "Zoom" stem from Nashbar that was marketed as a "road" stem with 26.0mm clamp, but it's clearly a 25.4mm clamp, and I'm using it with 25.4mm bars. It'll work with 26.0 b/c the removable 2-bolt faceplate, but the curvature of the faceplate and the stem clamp end line up cleanly with a 25.4mm bar and not with a 26.0mm bar.
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"c" is not a unit that measures tire width