[dostoyevsky]

I also wonder about this. On Facebook I sometimes see some Japanese native speakers write:

頑張って

whereas others will simply write

がんばって

I think the key is whether the hiragana version of the word is unambiguous enough for not being confused with something else that is pronounced the same. So I'd say the rule of thumb is something like: If it's a very common expression, hiragana should be specific enough.... Still confusing though: In my Assimil books I see 行きます but できます is always written in Hiragana (maybe the latter one cannot be expressed with Chinese characters?)