The first thing to think is whether your reader will be able to follow your system, and the second is whether they will appreciate it. Synonyms in brackets are fine, but too many will soon start to get in the way of the reading- most formatting should, as a general rule, be used sparingly. If you have too many brackets, then it might break up the flow of reading. It also assumes that the reader needs all this help, which they might not like- you know those pages on the web with lots of red and bold and words written in capital letters- don;'t they annoy you? That's the extreme, so try things, but in letter writing, I'd go for moderation. Use synonyms where necessary, but not too much.
