You'll note the three places I specifically mentioned as usually no problem are ones that don't require visas for Americans and which have very active land borders where vast numbers cross each day with no questions asked as to "ticket out."
Right, it makes sense that island countries would be tougher. Planning ahead shouldn't be too hard for the Philippines, Brunei, and/or Indonesia, none of which I plan to visit for more than a few days (if I go at all), but I'm hoping to have more flexibility with Cambodia, Vietnam, etc. Thanks.