First off, I would dig ALL repeatable high tones.
Second, if your discrimination setting is set higher than 0, you will not hear any low tones. Some iron will sound off as a low tone/ high tone signal, but since you cannot hear the low iron tone due to the disrim setting being set higher than 0, all you hear is the high tone.
If there is not an over abundance of iron when you are hunting, I suggest leaving the discrimination setting at 0 and listening to all signals. Also, very soft, faint low/high tones should be investigated. Some deep coins that I have dug gave the low tone/mid tone or low tone/high tone signal very soft and faint.
Third...don't use too much sensitivity in discrimination mode. Not sure where yours is set, but most CZ's go deep even with the sensitivity at 1 or 2. If they get pushed into the 7-8 range, most will false a lot. I can't run my 6a past 5 or 6 in most conditions.
Forth, make sure you ground balance properly and check it every so often.
Fifth, practice using the pin point button to 'size' the target. Lots of times iron will not pin point in the exact spot each time, and nails give a longer all metal sound when you sweep the coil using the pin point button. You have to sweep in multiple directions since you may be hitting the nail from the short side on one sweep, but you want to get it also from the front to back side so you can hear the longer sound. I may have not discribed this correctly but the main point is to sweep from several angles...on ALL targets.
One last thing...if you are strictly coin hunting in parks or where there is a lot of trash, seriously consider getting the small 5 inch coil. It can really get between the trash and find the coins.
Keep using it and learning it. Yes, most CZ's like iron and when I first got mine back in 93 I dug more nails the first 5 or 6 hunts than I can remember. Then I figured out that I should keep the discrim setting at 0 in most all hunting conditions unless there is tons of iron. Any repeatable high tone, especially faint and smooth, should be dug. I keep the volume above 5 in most cases since I want to hear the deep targets ring out. Sometimes I do like to use the modulation in trashy spots so I can distinguish the deeper targets.
Good luck and have fun.
JC