Good question! You have to read La bona lingvo de Claude Piron! I think it is the answer to your question.
Here's what I think: it is not bad to create new roots. It's actually a part of the development of the language (and of any living the language).
And I cannot stress that enough; it's all about roots. And with them and the mechanics of the language, you build new composite words.
So, first you have to check if the language really cannot deal with the concept you want to express. Do you really need new roots such as: print/i (presi), adher/i (alglui), maĵori/o (plimulto)? Each time we add a root, we make the language more complicated for new learners and we lose part of the magic of the language.
Then, if Esperanto cannot accomodate the concept, you can go ahead find a suitable new root, start using it and hope it catches on. Please define it first. Don't simply copy it, the rest of the speakers will need a definition, even if it is obvious in the other language. So, if you decide to start using spojler/o` (English: spoiler) please say what you understand by it. Or use malkaŝaĵo/malkaŝumo or intrigmalkaŝo.
We are a community of speakers, not a bunch of linguists in a lab creating a new language. For certain gramatical aspects, if you really think Esperanto got it wrong ... it might be too late to "fix" it. The only way out would be another language or project: Ido, Interlingua, Europanto ... There are many really good options out there. And remember that long ago people flocked from Volapük al Esperanto, maybe we'll do the same from Esperanto to your new language!
Having read La bona lingvo you can then see what some people have been doing to try to keep Esperanto bona. This site lists neologisms that could be built from the existing elements in the language and, (because sometimes new roots are actually needed) a list of neologisms that they consider are actually needed.
Don't go too hard on them, some proposals are great, some not, the idea is to keep the language in a sane state. Take what works for you and consider collaborating if you feel like it.
Just think that if we add a neologism each time we cannot name an object, or express a nuance that we have in our mother tongues ... Esperanto would be an unusable monster.
To finish the answer: it is appropriate to add new roots every now and then, but we, as community of speakers from different backgrounds, should be careful about it. Just how careful? That's what will measure the appropriateness of a certain neologism for each one of us. It's pretty personal.
People at the Akademio de Esperanto do part of that work for us. They'll monitor new roots and, if they deem them appropriate, after some (not necessarily short) time they will make them official. That happens in batches (the small numbers written by certain roots in Esperanto dictionnaries).