Donna J.,
Yes, much of the stuff that is published breaks the rules! But you probably don't want to give them a reason to turn you down. So, unless you have an exceedingly good reason that is essential to your plot/story, I'd avoid the rhyme no-nos as much as possible.
I have a rhyming PB that was requested by several houses and agents. Unfortunately, in my eagerness to get it out, I had it only minimally critiqued, though I did revise it a number of times. I received several long, personal responses, and each time I used the editors' suggestion(s) to improve the manuscript.
Unfortunately, now that it's finally polished and 'ready' (at least in my view), I'm out of appropriate houses to send to! Sometimes the rejections following a request for manuscript are even more painful, as it feels like the potential of the premise was there, and I just failed in the execution (quality writing).
I have now learned a very valuable lesson, both about having a manuscript thorougly critiqued during the revision process, AND about being too hasty to send out a manuscript. This one story is waiting on one more response before it goes into my 'if only' drawer (as in, if only I hadn't been such an DOLT, DUH!).
Nowadays, I get all my work critiqued thoroughly by a number of sources (experienced and not), and I still let it breathe, sometimes several times before sending it out. Honestly, this often makes it a six month process at a minimum for me.
I suspect there are other people that have a much easier time of it, but that's what has worked best for me.