I often trade SPX options. Lately, I've been trading skew and having decent results. However, in an effort to continue improving my trading, I'd love to hear any thoughts on peoples' preferred choice of volatility to plug into their model in order to delta hedge. I hedge with Black-Scholes-Merton. Yes I know its wrong, but in my experience if I account for the left tail and bound my downside risk, the model is still remarkably robust throughout most market conditions. The question is, do you use IV, actual vol, or some other number? Moreover, if you do use IV, do you use a "locked-in" IV - i.e. do you freeze the IV you purchased/sold each individual option at and compute deltas at that vol over the duration of the trade? Or do you let IV float and constantly update as the market moves?