It all depends on the searches you anticipate you'll want to do in the future. There is no right or wrong answer when you are keywording pictures for your personal library. Keywording for others, (e.g. a stock agency) is a separate issue.
So if you think you will need to locate pictures of grass fields in the future, as opposed, say to corn fields or wheat fields, then by all means keyword for that. There's no "too far" - a researcher may want to keyword the variety of grass in the field.
I've developed some rules for my particular interests. For example I keyword cars down to the "model" level but no further. "Ford", "Escort" but not "GL" or "Ghia". For airplanes "Boeing", "747" but not "436" or even "400".
For locations I will keyword to the street level in places I photograph often and to the city or province (state, county) level in places I visit less often. Ideally I'd replace all that with an accurate geotag and corresponding reverse-geocode data in the IPTC but that's another subject.
When you adopt a system that meets your needs consider that you'll need to apply it consistently maybe for years. So if your system is too detailed you may not have the time and energy to both keep it updated as your needs change and to apply it consistently. I find keywording new pictures quite satisfying but going back and changing keywords on old images very boring.
Regarding synonyms - I use them extensively in Lightroom as it saves a lot of time. I have guidelines for the synonyms I'll add when I create a new keyword. For example for Thailand I try to include the Thai script equivalent for place names. This helps when I export to Flickr so Thai people can more easily find my pictures. I also include common differences in transliteration from Thai to English - e.g. "Sukhumvit" and "Sukhumwit".
If you're interested I've written more about my approaches on my blog at http://bkkphotographer.wordpress.com/tag/keyword/
I hope this helps.