There's nothing right or wrong about doing it that way. I think about this topic from time to time as well. It comes down to two different aspects.
1. Practically speaking, what is the best way to manage my cashflow. There's alot of advice on this forum about keeping an "emergency fund" cash cushion. I think that's incredibly useful advice for anyone, particularly just starting out. MMM wrote an article about the costs of such a cushion, something like cash cushion vs. springy debt. His argument went something like, at some point, once you're...you know...rich...maybe it's OK to just have a floating low-interest line of credit you can pull on, like a Home Equity Line of Credit (HELOC).
2. What's my FIRE number really? If my annual spend is absolutely going to include an item, such as property taxes, then I include it in my annual spend. But other things are optional. Think of a home maintenance task like painting a bedroom. That isn't likely to be an urgent task. So it isn't a big deal to put my hands on some money to do that. If it blows my budget for this month, I can adjust some things, maybe do it next month. Once I'm FIRE, I can wait for a really good day in the market and sell a share of something. In any case, I'm not likely going to run into sequence of returns risk by having to sell something at a huge loss to pay for some paint.
But what if the maintenance item is a new roof? Well, that's where the HELOC comes in. I can get right to work on that roof, and then over the next few months figure out what to do about paying for it, either working a little bit part time (the horror) or adjusting down some spend in other areas. Or waiting for that really good day in the markets.
Hopefully the urgent need for a new roof is because an insurance-covered windstorm came in and so it doesn't matter, because most of the really big expenses I might slam into are covered by insurance. So there's a cap to the HELOC-expected expenses in a given year: my insurance deductible.
I don't know what the right answer is, and it varies by person. For my part, I am still working, so right now I cover these things out of new money, going into my second year of not keeping a cushion. Not sure the aggravation of having to keep such a close eye on the checking account is worth it, but it is an experience. Like when you were planning to sell some VTSAX to pay for property taxes and the markets have their worst opening week ever. Flexibility remains key.
As long as you don't forget that these bigger ticket, non-monthly costs are on the horizon, you should be OK. My annual spend number, and thus my FIRE number, include things like property taxes and insurance, as well as all my other monthly bills. But my budget as a whole is annual, not monthly.