I like to treat the synonym system as something that should be reactive in design. This design enforces the identification of a demonstrated need for the synonym, rather than proactive prediction of its potential for confusion. This stems from the fact that synonyms exist more as a mapping, not as a thesaurus.
Thus, the majority of synonyms tend to be handled as a side-effect of other processes, namely renaming tags or merging tags together. These usually are things that require a moderator to step in, as well as possible community discussion on the meta site, and so the reputation requirement gets a bit sidelined. But more importantly, these demonstrate that people are indeed, on the site, using an alternate term than the existing tag. Thus it is realistic that people actually might run into the wrong direction.
On Seasoned Advice, there is a tag [soda] that exists. "Pop" is another term for the same substance, but there is no tag synonym between the two. However, even though there are even questions that call it pop, the only tag they have is soda and there is no synonym. That's because even though there's a logical synonym here, there is no need to make a tag synonym because even those users will still tag it as "soda". This would be an example of the kind of scenario that a tag synonym isn't necessary - the community doesn't need to worry about pre-emptitively setting up a tag synonym because in practice, no one checks for a "pop" tag.
Meanwhile, on Game Development, there's a tag called [pathfinding], which has A* (written as [astar] and [a-star]) as its synonym. A* is a specific style of pathfinding, but the distinction isn't large enough to warrant a separate tag. Since both tags are in use, however, a synonym is helpful to setup so that the people who look for pathfinding stuff by searching for A* will find it.
Thus, to wrap this up, when trying to think of whether or not to implement a tag synonym, think about how people will use the tags over how the words are related to each other. If there are two terms for a place or period or methodology that comes up in Genealogy, if people actually are likely to use either term then a synonym is a good idea to setup. If there are two or more terms but one is fallen to disuse, is irregular for people to say, or if people just default to one term so much more, then a synonym probably isn't necessary.