As other comments in this thread document, various pronunciations of the letter 'r' can be understood, and have some level of acceptance. To answer Eey91's question above, very few Esperanto speakers trill the 'r' every time. Many speakers never trill the 'r'. There is no need to obsess about this sound, but if you want to aim for one of the most widely used pronunciations, aim for the Spanish single 'r', which has a single tap of the tongue, rather than a trill.
edward.qiq overstates the case, when he says "In Eo R could be pronunced anyway you want." Assuming that you want to be understood by the average speaker, then there are limits to how many pronunciations will make communication easy. He also misstates what PMEG advises. Here is a direct quote from that excellent reference: "Oni tamen zorgu, ke la R-sono ne konfuziĝu kun alia konsonanto, aŭ kun iu el la kvin vokaloj." Which translates as, "One should, however, be careful that the R-sound isn't confused with another consonant, or with one of the five vowels."
This might be due to the speed. For instance, in German r tends to be an approximant, but it will be trilled if a word is being emphasized for whatever reason. The two sounds aren't distinctive like k and t are, they just get used different times. Maybe in Esperanto tabs turn into trills when they're slow/emphasized/etc.
Mortto, you will find many people calling the 'r' sound a trill, and fewer, like Vortarulo above, who explain more accurately and completely about the single flap versus the trill. However, if you find any source saying that these two pronunciation variations for the 'r' constitute different phonemes in Esperanto, I would love to see the reference. That source is wrong. Esperanto has a single phoneme for the sound represented by the letter 'r', and moderate tolerance for various ways of pronouncing the letter.
A phoneme is a sound distinction which carries meaning. In a language like English or Esperanto, if you have two different phonemes, then you can find two different words that differ only by that phoneme, and have different meanings. Words distinguished in meaning by different pronunciations of the letter 'r' don't exist in Esperanto.
I guess if this pronunciation is being compared with Latin too. I have noticed that the single r in Latin has the same sound (a bit trilled) and rr even more. I'd hope if someone who knows Latin can affirm that. Also, I have heard this word viro with the pronunciation of r as in English (example, hero or aero) so perhaps it's possible to choice between Latin or English pronunciation.
There are no verb conjugations in Esperanto within a given tense or mood, but formal definitions of 'conjugation' include the variations across tense and mood. So the regular, consistent verb endings that we know and find so easy to make from any verb root, such as 'est-' [estas, estis, estos, estus, estu, esti], are a form of verb conjugation.