We have quite a bit of juniper gear, mostly EX series.
We have read and read/wrire snmp strings defined on ours.
Before v7 upgrade we had no issues with snmp tests to these devices from NCM, after the v7 upgrade we noticed that adding new ones and testing existing ones in the database were failing the snmp "test" button when managing nodes. The failure was only for the read/write string.
Here is what resolved this for us in case you run into this:
we had to add 2 extra lines to our juniper configs and the test succeeds after that, no idea why v7 requires this but it resolved it for us on all of our juniper gear.
the first 2 lines below are what were required to fix it, I added the template line for our read/write string as the third line for reference. Be sure to replace <read/write string> with your actual string 
set snmp view jweb-view-all oid .1 include
set snmp community <read/write string> view jweb-view-all
set snmp community <read/write string> authorization read-write
