Dumb question....  I understand that cruise altitude should be above 300ft. AGL,  even +500 going west and odd +500 going east.
  If I am planning a flight along rolling terrain, or flying say from flat land up over hilly, mountainous terrain where the ground elevation rises 500-1000ft. or more ASL,  do I constantly need to ensure my cruise altitude is adjusted?  Whats the easiest way to do this?  Can I use the elevation off a map, and use time/distance calcs along my route to know when to adjust?  Or is it less precise than that, and just pick a few prominent landmarks on my route and know to be at correct AGL at those?

In addition to what Brett said.  You'll plan your flight for a specific MSL altitude, not an AGL altitude, unless your need to fly at a specific AGL altitude for some reason.  For example, I live around some uneven terrain with hills that have bases around 900 ft MSL and tops around 1500 ft MSL.  When planning a flight, it's better to be conservative if you can, so your VFR cruising altitudes would start at 3000 ft AGL, which would be around 3900 AGL, meaning if I were flying west, I should fly at 4500 ft MSL, 6500 ft MSL, 8500 ft MSL, etc.  If I were flying east, I would fly at 5500 ft MSL, 7500 ft MSL, 9500 ft MSL, etc.  I'll also want to maintain my minimum altitudes over specific object, so say my route of flight avoids radio towers and other obstacles like that, but takes me over several small towns, some in valleys and some on tops of hills.  I would choose a cruise altitude at least 1000 ft above the highest town along the route, meaning I would have to fly at least 2500 ft MSL.  So, putting it all together.  Say I'm flying west, I could fly at any altitude between 2500 ft MSL and 3900 ft MSL, or 4500 ft MSL, 6500 ft MSL, 8500 ft MSL, etc.  You'll also want to choose an altitude that is high enough to give you time to react and altitude to glide in case of an emergency.

It sounds complicated, but it's not too bad in the end.  The most important thing to remember is the difference between AGL and MSL altitudes.  You'll rarely use AGL altitudes when flying, only to choose an altitude that avoids obstacles and is above your minimum altitude.  The rest of the time, you'll use MSL altitude because it is always the same, regardless of the terrain.