Generally speaking, in some ways it is important, because it shows past dating behaviors, etc. and helps you measure her/his thought pattern.
On the other hand, knowing too much about someone's past is never, ever good. It is given that you'll hear something that'll freak you out, or that you will see her/him do something and wonder, 'Is this the first sign of what she/he did last time?" or something like that.
Personally speaking, it's not important to me unless echoes from the past creeps into present and ruin it.