Always cover the short side of the options out first if you leg out. Never sell the long side first and keep the short side open because the risk is too high. Many platforms will allow you to close the whole spread down as a single operation, and that is fine.
A reason to leg out of the short side is that you can short again against your long options over and over without having to pay the commission of buying the long side of a vertical each trade as long as the underlying is trading in that same range. Many times I will let the long side expire worthless if I can daytrade the shorts against them multiple times during expiry week.
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