To add to Mike's comments about blade drift...the blade, itself, will contribute to this factor. Even if the blade stays relatively sharp for awhile, the set may eventually become uneven from just running on the tires, depending on the type of saw wheel design and blade. New blades tend to cut pretty straight...and gradually that changes. You also want to dedicate any blades for resaw and other straight cutting work...with not even the "hint" of a curve. This will help minimize blade drift.
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The most expensive tool is the one you buy "cheaply" and often...