As I have been sewing this week (a Lot as you can see from my yardage used). I have been thinking about thread. I have family & friends who pre-buy their thread-- by the truck load... I used to do that-- But have not in years. I certainly try to get a sale and buy it them-- but only what I can use in a short amount of time.
Cotton thread Rots... There is no point in buying a lot ahead. (for me). And those giant spools.. I don't tend to buy them. I sew a lot- but I don't sew enough to use them up in a short amount of time. I would throw away more thread than I would use.
You can easily tell if it is any good or not.. unroll say a foot and tug it-- does it break easily? or stay strong? If it breaks- does it leave fine strands in the breakage? It is rotted.
And sure you can still use it--even rotted... but do not be surprised if your sewing machine then has issues... like breaking thread, knotting in the bobbin case...tension issues. I can't have that...And then you just spent all this time making your creation... do you want to deal with poor thread quality and worry about threads breaking after washing and use? I have gotten things as gifts and they have literally come apart in the wash due to poor thread quality and poor tension.
I think thread brand is personal preference. I have used some very pricey threads that don't sew any better than some of the cheaper ones. Right now I am into a Poly-Cotton blend.. They seem to hold up very well and sew nicely. I like them for my dog beds where I am sewing with heavier duck cloth, yet they also do nicely for baby quilts. I also have 100% cotton threads here and I do use them some. Different thread weights for different jobs.
I do have to say I totally stay away from the very cheap threads. The ones that are in bins in the store- marked $1.00/spool. I have not had good luck with them at all. My threads are all brand names.
And yes, I do buy thread more than the average person- I use it faster maybe-- and I don't overbuy. I can generally find it on sale someplace or even buy it on eBay. And sometimes- I just have to pay full price and live with it. It averages out. LOL.