Full key space coverage when generating a rainbow table

For example, say we want a rainbow table that supports MD5 for all lowercase alphanumeric of length 1-8.

For such a table, does the generator stop creating new chains when it knows for sure that all a-z0-9 possibilities are encoded in the chains? Perhaps there's some special choice of reduction function(s) and chain seeds that can be proven to cover the desired key space.

Or maybe the generator takes some probabilistic approach and stops when it is fairly certain that the entire desired key space is covered?

How do rainbow table generators make this decision in practice?