What is the best way to categorize saving for a "real expense" which may come intermittently, but is more or less predictable. Things come to mind such as HOA expenses (quarterly) or car taxes (annually). Do you set a monthly budget for this or set a savings goal by the time it's due? Thanks.
It's really up to personal preference, but if you set a savings goal for when it's due, YNAB will automatically do the math for you on how much you need to budget each month to meet the goal. And then if one month you can't meet the entire amount, the next month YNAB will adjust. So say in 3 months $300 is due. YNAB will tell you you need to budget $100 each month to meet the goal. Then, if you can't budget that $100 this month, next month it will tell you you need to budget $150 to meet the goal.
Most long-term YNABers try to standardize our budgets as much as possible, so that all savings is added to equally and so that changes in income (up or down) can be dealt with efficiently. My car registration is only $75 annually but I save for it $6 per month. This makes all my saving and all my spending totally predictable out of my income, and I'm never left trying to scramble to quickly add $50 to it at the last minute.
$6/mo is an extreme example, and on any given month, I could easily absorb $75, but it's nice to not have to.
Also I realize I've done the usual thing of answering the philosophical question instead of the transactional question. Honestly, figure out how you're going to save first, and then use the appropriate goal for that method. Goals should come second; your plan should come first.
Goals exist to save a tiny bit of effort. Either of the two you mention would work (although you have to compute the value for the Monthly Goal), as would just typing in a budget entry. Most people would use a Save By goal.
You're the only one who knows if you'll budget to the category in the month of the outflow before making the payment. If you cannot do that, then you'll want to plan to have funds available in the month before the outflow. Set your target date on the Goal accordingly.
Rule 2 would have you save up in the months preceding the outflow. As WordTenor indicates, using consistent amounts each month makes the entire budgeting process much easier and significantly reduces the chance of making mistakes.