Right now I cash flow so much per month from my normal job (plus rentals as a bonus) that I don't worry too much about reserves
I'm on the same boat myself, however, I like keeping my finances separate from rentals' income/expenses. Obviously, if something crazy happens, I would definitely use my money to pay for it, but assuming no major repair exceeds $5000, I shouldn't need to "touch" my own money.
Well 5k$ is better than zero, but it is still an arbitrary number. The right amount depends on the type and size of the property, the minimal expenses to maintain it, etc.
In my parts, windows and doors are good for 15 years (more or less) and roofing is typically good for 20-25 years (do not expect shingles to last longer than the guarantee), so you set aside enough over time to make the next changes a scheduled expense instead of a problem that makes a hole in your cashflow. Also, a big bill divided by 15 or 25 years is suddenly a minuscule amount, and is still yours until the bill comes :)
Next, I will clarify "regular surprises": faucets will leak. Tenants will ask for new locks, electric sockets, light fixtures, etc. once in a while because they are no good anymore. Keep track of these "one-time" expenses and make sure to budget for them. If for a given year you don't have that much spent, adjust the mean amount and then you can use the difference for something else.
Do not underestimate the power of vacancies. This is the place where you transform your landlord life by removing a regular source of stress. 5% vacancy is about a month every two years, which is not much if you have a SFH. Vacancies are caused by time between tenants of course, but also when you must do renovations and can't possibly have tenants around or you have to give them a rent holiday as a compensation for the inconvenience. Vacancies are also caused by tenants not paying for any reason (aka. deadbeats), and this occurence is never limited to a month at a time. If you have accumulated that 5% and have a stellar client for 2-4 years, you will have some time to spruce up the place before renting it again, thus getting a better rent from the next tenant (and compounding the win!).
Take the time to estimate the above factors BEFORE taking money out from your landlording business. In the case of multi-unit buildings and it takes experience before you can eyeball those numbers, so sit down and do some number-crunching. Try to know how long since the roof, furnace, windows and other big-ticket items have been replaced, find out how long is their remaining service life and get price estimates for their replacement by professionals. If you get to DIY a job, pay yourself the difference when it is done for being a badass :)
Now, some here seem eye these funds for spending because they think it is "sleeping money". Nothing is stopping you from investing it, IF you do it in such a way that it will be avaliable in advance. Taxes? use a high-interest savings account. Vacancies? depends on your property, but I like to keep that in the savings. Your roof is due in 5 years or more? Balanced portfolio.
I have partners, so keeping finances separate is a requisite for me; if you are the captain of your own ship, doing the same will trick you in thinking that your cashflow is lower than reality in good years and unaffected by bad years except if you have a REAL surprise (flood? tornado? act of God?). I consider that most "surprises" experienced by landlords are entirely previsible.