I have sewn on my layouts and even sewn the title on one of my pages. Not really concerned about threads being acidic --- most are nylon or polyester.
I have a few pix in my gallery --- check on page 7.
If you are worried about it, prewash the fabric to remove the sizing. That would be the most likely culprit if anything was going to be a problem. I sew fabric swatches to my layouts when it is about sewing, hasn't been a problem, but I really have not thought long term...
I make flowers for my cards mostly from fabric & laces....and I usually sew on every card I make with cotton threads..mostly. Always adding laces..But like I said, I make cards & journals, mini books, stuff like that.