Negative. You can "buy down" the rate by paying points (usually 1% of the purchase price = one point). If you are worried about rate.
Here's my theory (and I'm sure others will agree or disagree) but I like no money down interest only loans. The difference I would use in the payment between the interest only and the 30 year fixed, I would then take and invest in something that gets me a safe ROR say around 8%.
My theory...Equity in a home is truly nothing more than a number on paper. If you take that $60K you'd put down on a $300K house and have that liquid, as opposed to using it in the down payment, you can access it anytime.
However, if that $60K is sitting in equity in your home and say something bad happens like you lose your job, you aren't going to be able to touch that money. The bank wouldn't loan to a person who is unemployed. So I'd rather have the money working for me in an accessible account because IF you did lose your job, you'd have the $60K to cover your bills for several months or years (depending on what your actual outlay is of course).
Make sense? Hopefully I explained it correctly. I think others will disagree but I believe in leveraging my money.