You can't say "require to be", probably because "require" basically combines the verbs "need" and "to be" on its own. "They require tuning" or "Tuning is required" are fine.
You're using it fine yeah. I wouldn't say it's 100% interchangeable, "most cases" literally refers to repeated occurrences and suggests that the majority of them will be the same, but "most of the time" could refer to the majority of all time or a single time period.