Is it necessary to learn all of the synonyms in Esperanto, like plaĝo/strando, or razeno/gazono/herbotapiŝo? sablohorloĝo/klepsidro is another one. Is there a list somewhere of which is the most commonly used in each pair?
Ultimately it is up to every speaker to choose how to express themself. Naturally sticking to commonly accepted rules and words helps to getting understood.
The highest authority is Akademio de Esperanto, which gives recommendations of language use. The foundation is of course La Fundamento laid by Zamenhof himself. A good dictionary has a marking (e.g. a star in PIV) for those fundamentaj words.
There is also a site La bona lingvo, which have lists of words what they consider as preferred ones. Those lists are based on those fundamental words, their derivations and use by respected authors.
There are way too many words in Esperanto to memorise, so choose what to use. But if you choose a word that deviates from the one given in the sources above, be ready to clarify and explain.