Is it okay to add time for "unexpected delays" to a time estimate?

Is it okay to add time for "unexpected delays" to a time estimate?

I've landed my first contract (hooray, self employment!) and the company
is asking for time estimates.
Programmers are notoriously bad at time estimates, and I know I've been
laughably wrong before. It's fixed bid, so I'm not concerned about
charging. I'm just concerned about expectation management.
So far I've itemized the work I need to do, estimated the time it would
take, then significantly padded that time. I'm still nervous about it,
though. Is it acceptable to write in time for "unexpected delays"? I've
estimated 4 weeks, and I'd like to add a 5th week for problems I haven't
thought of. Is that something people do? Would you balk if someone gave
you an estimate with that in it?