If it will cover your costs and allow you to use it for free at today's price then I wouldn't worry about small market fluctuations.

The most dangerous thing you can do with cash flow is "play with tight margins"   like purchasing a house for 275k while the rent is below  aprox 2900$/m and thats roughly ... as i would never risk that much money on high end rental because they fluctuate too much  (nearby developments, markets going up and down, someone decided to raise taxes again ...etc)cuz just imagine ...today  you are going to make 2700/m from 275k  which roughly means your cash flow is going to be ... $150-200 bucks a month after everything ( PM, taxes, P&I, some other Opex and Capex)  and dont quote me on that exact number  its just meant to show how tight of a margin you will get.   

Cash flow can fluctuate.

The housing market will always fluctuate in price, but it will always correct itself.

Or I should stick to the existing 30-year fixed mortgage to avoid fluctuations in the monthly payment to keep it for retirement?

We don't get the crazy fluctuations of the Californias, Colorados, ETC..Just my 2 cents..

And banks may fluctuate between paying 75% LTV down to 70-65% where 20% down is pretty consistent.

My credit score is and was around 800 (fluctuates because I am doing a 0% interest card for grad school, where my company pays me back at the end of the semester).

However, your rate would likely be the market rate which could fluctuate wildly over the next few months.

I would recommend that for a few reasons: 1) gives you time to gather data on what your average monthly utility costs are just in case they fluctuate (like mine) 2) while avoiding over or under charging the roommates.