Hello
In SRX the Source and destination NAT are seperated and looked at different stages in FLOW ( attached ) .
So the destination NAT hits first in flow , then route --> zone --> policy --> then comes source NAT .
This is as per Junos design that the 2 types of NAT lookup happenes in 2 stages of flow .And they are diffenet from policy lookup , unlike in SSG where NAT happenes in policy itself .