I keep a "miscellaneous income" tab on my budget. Anytime I sell something on craigslist, or engage in any of my side hustles, it gets accounted for in that category. Also any one-off windfalls go there. It all gets accounted for, and in a way that states it isn't predictable. My only exception to this is gift cards, because they are more or less earmarked for spending. If we are given a gift card, we just spend it, and only account for the extra balance paid over the gift card amount in-budget.
But I'm a person who loves budgets, and has a handful of random side hustles that are nice to track. If you don't want to track it, it makes less sense to designate it.