Don't use them! (Just a suggestion, but a good one.)
Do you see the difference between [Client First-Name] and [Client First–Name] (It's a tough one. The first variable uses an 'en' dash and the second an 'em' dash).
How about [Client's First Name] and [Client’s First Name]. Check out the 'angle' of the apostrophe. One is 'straight' and the other is 'curly' (Microsoft's description, not mine). Quotation marks are just as bad.
There are (at least) 4 different kinds of dashes, and 3 different types of quotes and apostrophes that Word recognizes. When Pathagoras searching for a match, it is looking for only one of those -- the one you typed in your variable. If you are not consistent from document to document as to the type of character you used in each, you will may not be successful in properly replacing all variables. (Now, to be fair to Pathagoras, it does try to adjust for these known character differences, but there may be others lurking out there that we have yet to discover.)
Our suggestion: Don't use dashes or apostrophes or quotation marks, and similar characters, in your variables. You (and your staff) likely will be just as happy using [Client First Name] in lieu of [Client's First-Name]. The same message is communicated, with fewer characters to type and to remember.