Chris,
Hunting in Iron Mask (especially at the lowest levels, such as, -16 to -12 or so) is mainly used when you want to "maximize" your finds especially in areas where you know old targets exist. It's kind of like hunting in all metal but you have both audio and meter ID to assist you ... both of which can be powerful tools. However, most people expect far too much from detector ID systems in my opinion. There are just too many variables so one shouldn't expect perfect ID'ing every time. Try to constantly experiment in the field to improve in this regard.
The constant sounds "can" be annoying (so you should do it when you are in a good frame of mind) but for many people these sounds can become sort of like "music" to your ears after awhile ;^). So it's worth trying as much as possible. Keeping iron mask wide open will get you targets you would otherwise miss due to masking, etc. It is also a good way to learn more about your detector as digging all or most targets (good and bad) and relating them to the audio response and meter reading is the best practice possible with any detector.
Disc settings, especially at high levels, are more often than not used in special situations ... for example, you don't have a lot of time, you have a headache, you're trying to find (or not find) a specific target, etc.
Most good targets are repeatable so you should definitely look for that type of response, but there are many exceptions, ie Coins on edge will often read better in one direction than another, odd shaped objects, etc. Once you get a small signal, target profiling - which involves circling the target and swinging over it at many angles , raising and/or lowering your coil, determining if signal is smooth/raspy/etc, and so on - can all help you extract information you can use to decide whether to dig or not. Ultimately the only sure way to determine what a target is would be to dig it. You definitely get better with practice.
Without getting into any what's the deepest arguments ... if you haven't found any deep targets with the Explorer you either haven't swung it over a deep target yet or you are not listening properly because the Explorer will find very deep targets.
Please don't get discouraged as it is a great detector!!!
Hope this helps a little ... Gord.