Lee Precision tries to live up to their name and produce bullets that are the size advertised. Most of the time, they drop from the mould of such a size that they don't need to be sized, just lubed.
Invariably, some alloys will come out undersized a bit.
However, it is my understanding that Lee will replace moulds that drop undersized bullets for free.
I cannot attest to that first-hand, but they did replace my priming arm, no questions asked.