Many of users here are reporting admob open and exit interstitials …One thing I can say to you all is just forget using admob for open and exit interstitials for your apps…if you have more than one screen in your app you can combine admob with other networks for open and exits…If you have single screen apps its best to use a admob banner and exit interstitials network with high eCpm or mediation… it’s worked well for me…There are some cases you can use admob in exit ads if your ctr is low…One thing I learned from opening ads is that you get good revenues but you loss active users…users don’t like to see ads popping up when they open an app .Banners are fine they are good they take less space and not intrusive… Exit ads are another thing ( ex : if user completed an action in your app and he wants to leave your app you are suggesting him/she some new apps so there is a huge chance for conversion so high conversion rates high revenue)…Its just my theory how to improve performance… Hope you guys get some info from here 
but all depends on some particular factors
Exit ads is against admob policy, but you can use other ad networks. On launch ads is Ok, users don’t like ads generally, where you put it doesn’t really matter.
If you talk about conversion, asking user before showing ads (like appbrain) will get higher conversion, but lower revenue due to lower impressions. Higher conversion is not necessary higher revenue. Using exit ads just increase your impressions thus increase your revenue. So using it in combination with on launch ads will increase your revenue further.