When the hiragana that follows the kanji is okurigana (する at the end of a verb doesn't really count as okurigana), then yes, generally you would use the kunyomi. But there are always exceptions. Off the top of my head I can think of 存じる (honorific form of the verb "to know"). I know there are a lot more, but I'm drawing a blank. As for kanji compounds, they usually use the onyomi, but again there are a lot of exceptions. For example, the word for "exit," 出入口 (pronounced "deiriguchi" and not "shutsu-nyuu-kou"). And this post. That's just one of those weird exceptions that breaks away from all the conventional rules. Another one would be 煙草. If you pronounce it using the onyomi, it would be "ensou" (literally "smoke grass"). But the correct pronunciation for it is "tabako" (tobacco). And another one would be 海老, which is pronounced "ebi," not "kairou." In fact, a lot of foods have unique kanji pronunciations, like sushi "寿司."