Hi, Frends
In PropellerAds, which kind of users do you focus on?
Just High, Or High & Medium & Low, or High & Medium?
I always tested all the types, but usually the High category produced the best results, on the other hand, the "low" cat often sends the most traffic, so even though the ROI is lower there, the volume can make up for it. I also recommend to target each of them in a separate campaign.
Hi Dollar,
The high / Medium / Low categories are related to the user response towards the ads in general. That means that the high users tend to interact more than the medium or low ones. Nevertheless, it causes some differences in your bids and so, as said Matuloo, it's better for you to test each and every category. Regarding your verticals, you can have a better ROI on low or medium categories. It all comes to the balance you find between high interaction rates and a higher cost by interaction.
I also recommend to separate your campaigns between the categories, to find your sweat spot.
Greg - Spicyoffers
You have to test, but for new campaigns I would start with HIGH and once I'm profitable and want to scale things up I would go to MEDIUM and later maybe even LOW (though that's totally trash)

For a lower payout offer I usually choose all since it doesn't matter if I don't get volume. For a high payout offer then I would click high first and then scale to others (just like others mentioned)