I tend to let the words come, although I'm much more likely to use rhymes if I'm writing lyrics without a melody in mind. Both can work, but a mix of non rhyming and rhyming can be quite effective. For example, one where Deadwood and I wrote versus and chorus independently and then glued them together with a prechorus:
You drew a line
Now I'm standing on the edge
of reason. The edge of reason.
You cast the dice,
And you gambled everything.
Was it worth it? Was it worth it?
'Cause nothing's changed.
We're still the same.
There's nothing here,
Nowhere to turn,
And no way back again
If you're ready, we can start again.
We can start again, if you're ready.
We can start again. We can start again.
That probably breaks a dozen rules, but put in the right musical context, works somehow.