I think an all-or-nothing approach is better, honestly. Blends and 100% cotton behave differently in terms of wear, stretch and shrinkage. I'd be concerned about the two not playing well together if you mix them together.
But I agree with skydiver70 that it depends on the recipient. A buyer will probably expect a more traditional approach, which means traditional materials. Loved ones (or yourself :)) will be more forgiving.