To LF - I have never looked at Globex levels. So, I could be missing something relevant and helpful there.
IMHO, there are Four ways to think intuitively about importance of globex levels:
1) ES is a derivative of stocks, so stock RTH hours should be most important
2) However, there is a correlation between global markets. So, what happens during Europe and Asia does get reflected in ES price and those levels should have importance.
3) Most of important news like earnings, frauds etc. on US stocks get announced pre or post market. If a significant news for a major stock like AAPL, MSFT, CAT, JPM etc. gets announced, then it should affect the whole sector, which would affect ES price. So, important levels of ES that get established due to stock specific news or geopolitical news during pre or post market price level should have significance.
4) ES Liquidity is smaller during Globex compared to RTH, so stop runs etc. should be easier to implement during Globex. This means Globex levels should not have much significance.
Now, all the above is hypothetical things that come to mind on thinking. Whether globex levels have any statistical validity or not can only be established after some analysis.