My son's endo has been checking his androstenidione and plasma renin levels with every blood draw (about every 3 months) since he was diagnosed at 2 weeks of age.  Is your child a salt-waster?  I assume so since he is checking blood sodium, but if not I don't think it is as important to check the plasma renin. 

I think the advantage of the androstenidione (and please anyone feel free to correct me if I'm wrong) is that it gives a longer term picture of how good the medication is suppressing the hormones.  The 17-OHP is more influenced by things like recent illnesses, stressful blood draws, and how recently the person being tested took his/her medication. 

I think the plasma renin is really important to check based on my son's experience.  About a year or so ago, the endo cut back his florinef from 2 tablets/day to 1 because his blood pressure was elevated.  As it turned out, he needed a lot more florinef than that!  This lack of florinef caused his plasma renin AND his 17-OHP and androstenidione to shoot way out of control.  By increasing the florinef back up to 2 tablets/day, everything went back to normal.  So if you have an increased 17-OHP it's not always because of too little hydrocortisone.  My doctor never checks my son's blood sodium level so I don't know how that would have correlated with the elevated plasma renin. 

Some reasons why you doctor doesn't check everything may be that it requires more blood and costs quite a bit more than just doing the 17-OHP. 

As far as the best time to have the blood drawn I believe that most people agree that it is best to draw the blood in the morning, before the first dose of hydrocortisone.  Not all endo's think this is that imporant though.

All endo's seem to have their own preferences for how to do things.