Always spend bitcoin if they accept Bitcoin for something you're purchasing. Money spent is money spent. Doesn't matter what kind of money it is. If you used dollars instead of Bitcoin you could have just as easily spent those dollars on Bitcoin instead. But if you prefer to keep a little local currency for ease of use, then when a merchant is selling an item you want and they accept Bitcoin, spend and replace. Saving is saving. Spending is spending. Doesn't matter what you spend. Matters a lot what you save.
Discussion
I gotta start getting in this habit then, of replacing when I spend, been too busy saving my time in bitcoin
If you can't afford to replace the Bitcoin you're spending, then you don't need the item. You can still get it if you want, but if you didn't Bitcoin and don't replace it, then you're spending savings. Which is fine, that's what savings is for ultimately. But I would recommend to only spend savings on things that improve your life in a meaningful way, like vacation, travel, or a productive asset like starting a business, or great utilies like a home or a car.