I picked up the following phrase to say "I hope you are well":
Looking this up in a dictionary, it seems it could become:
Is this correct, or is it better to leave out the Kanji-fication?
A rough¹ count of the results in the Balanced Corpus of Contemporary Written Japanese (少納言) gives the following numbers
上手くいく 274 8.2% 上手く行く 93 2.8% うまくいく 2742 82.4% うまく行く 220 6.6%
leaving out rare (<1%) kanjifications such as 旨く行く.
¹ I checked い・く and 行・く for the following conjugations: ～く、～か、～っ、～き, but didn't check the individual results for false positives.
These numbers clearly suggest Not To Kanji for うまくいく (as a general rule — writing by numbers is just like painting by numbers, i.e. not the height of the art).
うまい itself has slightly different numbers, but still suggesting Not To Kanji:
うまい 11153 79% 上手い 1837 13% 旨い 665 5% 巧い 238 2% 美い 167 1% ウマい 58 0%
Personally I don't think it makes a huge difference to use kanji or not for short phrases like that, but its good to use kanji if you have a massive wall of text because it can make it easier to digest and you can get the meaning quicker than by parsing every hiragana character one by one into words
It depends on your audience.
For anything longer than a simple phrase, the only place you'll see all hiragana is in children's books. But newspapers use more kanji than most people sending IMs to each other. Even then it can depend on the person. And not just education level, but individual idiosyncrasies.
Really, it comes down to experience. You send and receive enough messages until you get a sense for what words are not written in kanji even though they exist, what words are almost always written in kanji, and what is considered a good balance between kanji and hiragana when a lot of the words could go either way (think daily conversation level).