substr function with termination character

No, you should not need to quote the /. In general, the only time you need to quote something is if it contains spaces.

However, I often find myself quoting strings to help remind me that it is a string literal vs. a Tcl command.

Also note that using braces {} often accomplishes the same effect as quoting, but with a subtle difference. Braces create a Tcl list, which when referred to as a complete entity appears the same as a quoted string. So, in the example below, the two forms are generally equivalent unless a particular command is expecting a list, then the latter form is required:

 
 set bar "This is a string" 
 set foo {This is a list} 
 

However, there is a subtle performance difference between the two. The list causes Tcl to allocate a list object containing each list element in its own a seperate object. So, though the intended result appears the same, the list form has a larger impact on performance.

Hope this helps.