There are two ways to turn off welcome messages:

By component parameter

By add

$COMPONENT_NAME/welcome-message[B]=false

where $COMPONENT_NAME is name of MUC component

By default room config

There is new field in room config:

<field var='tigase#welcome_messages'
       type='boolean'
       label='Send welcome messages on room creation'>
  <value>false</value>
</field>

If it will be set to false in default room config, all newly created will have disabled welcome messages.

By default welcome messages are enabled.