Obviously you don't have to worry about earnings when trading QQQ and IWM. And if you did it on stocks before earnings, you just want to decide if you think the change in front/back month skews would be greater than the decrease in IV of the back month. Capturing this skew in the AM after earnings can sometimes easily outweigh the loss from back month IV drop. It really just comes down to capital requirements. If your short contract is exercised and you have the capital, great, you are synthetically the same but now you have captured the remaining time from the exercised contract. If, on the other hand, you don't want to have to think about capital being tied up and there isn't a big skew advantage with ITM, just go OTM on both sides. I find that when you have a lot of these on at once it can just make things a bit easier. Start with the pennypilotlist if you plan to use this strategy on stocks. And watch out for the Back-2-Back low debit calendars or commissions will run away with your profit. Another trick, if you are worried about IV, is to place a butterfly in the middle before the EA. This will push the middle of your risk graph out in your favor and reduce IV risk.
Thanks ExchangeRider and qqqoptions. FYI: the ThinkorSwim chat rooms blow this site away for useful responses.
If you're wondering about calendar spreads and inexperienced... take my advice and do plain (call or put) spreads instead. And stay away from credit spreads as they take alot more discipline and knowledge of the market. Calendar's are alot more difficult to time then plain call spreads and you'll end up making alot fewer trades which means less slippage. The farther out you go the less likely you'll lose all your money as you'll trade less ~ GL
Ever since ForexForex left the ET option forums has gone down a notch in usefulness, but if you block out the noise there is still some good advice here. EDIT: Another missing ET member is spindr0.