You have to be concerned about compatibility. If you want to use glass from different manufacturers, you have to test the combination yourself. You will need to determine the annealing point(s) and observe them. You need to ensure that your annealing is adequate, so you can distinguish annealing from incompatibility stress. You will need polarised filters to determine what degree of stress is in the fired piece. This takes time. The manufacturer has already done it for you and this is reflected in the cost and ease of use.
It has been reported that float glass from different float manufacturers has proved incompatible. Certainly anything other than bottle glass and float glass will be incompatible, and most of the bottle glass prooves to be incompatible with float. Float is a relatively difficult glass to work with.