I would hesitate using sideless bobbins. I have read that the lint that falls off can get into the case and it does seem reasonable that you would have some particles of glue falling off. Thread wouldn't hold by magic, they have to use some sort of adhesive to keep the thread stable. I only use the plastic cases. I buy them in fairly large quantity and they really are reasonable. I'm a great believer in preventing the great-'oh, no, it's got to go into the shop' routine.