Quote from hedgeman:
Don't forget, these back ratios will have the same risk as standard credit spreads over time and thats why it is not wise to be sitting in these trades or any as expiration nears. This is how you can lose big and why a lot of traders get lit up.
On the other side you won't see much profit (if any) if you close those spreads early. The alternative strategy would be to roll spreads a bit further from ATM, buy back some shorts and buy more longs as expiration approaches. Maintain the ratio as if the backspread was ATM. i.e. take gamma ratio of ATM and near ATM with the same strike gap. This will limit your losses if the underlying gaps to your short strike.
I would still close out all shorts a day before expiration just for peace of mind. But let longs run, they all would cost you a penny.