I'm a 7th level CC wizard, and still trying to get the hang of the metamagic feats. Are the following things true?

* Heighten doesn't affect spells which are already the highest level you can cast (for me, 4th level spells)
* Heighten doesn't affect spells which already factor caster level into their effect (MM, Burning hands, Niacs)
* Heighten doesn't cost any additional spell points if it doesn't affect the spell
* If Heighten affects the spell, it makes the spell cost the same as the highest level you can cast; therefore, it's 'cheaper' to heighten a 3rd level spell to 4th level than it is to heighten a 1st level spell to 4th level.
* Heighten raises the DC for resistable spells

If all of these are true, is there any time that I wouldn't want heighten on? I figure that if I'm casting a 1st level spell that can be heightened on a ~CR 7 mob (e.g. hypnotism), it's probably not going to hit unless I heighten it. I suppose if I went into a low-level dungeon, it would be a waste to heighten spells which would already hit the mobs without it...but I don't ever do that. So should I just leave it on all the time?