do we need to "unset" variables in TCL?

It's possible to determine whether a variable exists before using it, using the info exists command. Be sure that if you're not using unset, that you don't upset the logic of the program somewhere else.

There's no Tcl-specific reason to unset a variable, that is, it's not going to cause a memory leak or run out of variable handles or anything crazy like that. Using unset may be a defensive programming practice, because it prevents future use of a variable after it's no longer relevant. Without knowing more about the exact code you're working with, it's hard to give more detailed info.

In addition to the other responses, if your Tcl version is new enough, you can also use:

unset -nocomplain foo

That'll unset foo if it exists, but won't complain if it doesn't.