Welcoming new users onto Wiktionary helps build the Wiktionary community, and breaks the ice for leery newcomers.
In general, sysops do not 'welcome' new users when the account is created. For registered users, one or two good edits is a rule of thumb (so as to not waste time on vandal/sockpuppet accounts.) For anonymous IP contributions, most sysops wait for 3-5 'good' entries.
Besides giving new users helpful information on how to find their way around Wiktionary, the welcome messages change the display for those contributions, making "recent changes patrolling" slightly easier, when used correctly.
There are three general ways to welcome new users, depending on how they have conducted themselves.
The template {{pediawelcome}} is used for users who obviously have contributed to Wikipedia; they may or may not have some preconceptions about how things should work.
The {{welcome}} template is the general purpose template for new user accounts.
The template {{welcomeip}} is for non-vandal IP accounts. Earlier versions of this template simply restated the request to create an account many times.
Note that vandals are only 'welcomed' by accident, when following the above rules of thumb.
Adding the comment "Welcome" or "Welcome, please consider creating an account" in the edit summary is common.