It is not standard, but I do believe it is fairest to both parents. In many states the standard is to readjust child support every 3 years OR when there has been a substantial change (such as 20%) in income. However, without exchanging tax returns how do you KNOW whether there has been a 20% change in the other person's income?
Most state laws are written to say that children are entitled to be supported by their parents, and entitled to more support if the parent or parents make more. Being transparent with returns helps ensure that the children are not being denied support they are entitled to.
Similarly if the NCP's income goes down, the annual adjustment gives an immediate decrease in the child support payment (fairest to NCP) rather than having to wait for 3 years.
In my state, you do not need to go back to court if you can't agree on a child support adjustment. It is handled by the child support enforcement agency. My divorce decree specifies this is the procedure.
Possibly you don't completely trust your Ex to give you his/her REAL return, as opposed to one they print at home with numbers picked out of thin air. The IRS allows a trust but verify alternative: Form 4506T-EZ allows your Ex to request a free transcript of their tax filing and have it sent directly to you. And vice versa.
Regarding the deductions from income that the federal tax law allows - 401K, insurance, etc. - one possible way around that is to use the income amount on the W2 that was reported as state or local income. In my state they don't allow deductions from taxable income the way the federal government does. Another way around that is that the W2 does at least report the 401K amount so you can add that back in. Health insurance deductions are shown on some paystubs so the final one could be exchanged if they show year-to-date deduductions.
I agree that it is not common to share tax returns annually with the Ex. Many people would prefer to show their returns to the state bureaucracy than to their ex-spouse. However my preference is to deal directly with my Ex.
I never "meet" with my Ex to discuss child support. I email him a spreadsheet with my calculation, and he either agrees or sends back a different spreadsheet with an explanation of what he thinks I miscalculated.