I found one thing which we can actually omit:
If your server is new enough (so implements RFC 6121 correctly), you can skip a roundtrip by skipping the urn:ietf:params:xml:ns:xmpp-session IQ.
The Extensible Messaging and Presence Protocol (XMPP) historically had a Session Establishment request defined in RFC 3921 which clients were required to perform at the beginning of a session. RFC 6121 dropped this entirely. This specification reinstates it as an optional no-op to aid backwards compability, matching commonly deployed workarounds.
and this is also mentioned here https://tools.ietf.org/html/rfc6121#page-112
Can we skip this step with Smack?