Is there a some general rule when to write 物/もの in kanji or hiragana?

Question

So far my perception had been that you write もの when it is used for grammatical purposes, and 物 when an actual object is meant, but I've encountered many situation where even in the second case it is written in hiragana.

For example:

「これは、いいものを見つけたよ。」

or

「これはいい物を見つけたよ。」

Answer 1

「モノ」「もの」「物」, There is a reason to use for each of them in many situation, but few people use them consciously. Therefore, there are many people who use them incorrectly. Some people use hiragana to soften the mood of a sentence.

there are general rule.

• モノ： When it includes other meanings as well as the meaning of the kanji
  例：ヒト・モノ・カネ

• もの： When used as abstract matters
  例：なにか温かいもの

• 物： When it is visible substances
  例：箱の中の物

The following paper on katakana notation may be helpful.
カタカナ表記語の意味についての一考察