Rawr is a great program, but it's really meant for top end optimizing. If you just dinged 80 and you're starting heroics and 10 man naxx now, it's not going to help as much. Every time you get a new piece of gear, it's going to change your current stats, obviously. Every time this happens, rawr is going to recalculate your optimized gear, and will tell you different things than it did previously. It's really best to just look at it as a suggestion up until you're at the point where you're fairly well geared, and you're looking to see if one piece is an upgrade over another.
For example: using arbitrary numbers that have no real meaning
Your current:
int is 1000
spirit 600
crit rating 20
spell power 1500
Lets say you get a trinket putting your spell power up to 2000 while the rest of your stats stay the same. Rawr is now going to recalculate all of your stats, and maybe will suggest items that will boost your int next. After you get an int item, it will re-optimize for a different stat or maybe back to spell power. This can make it very difficult to see what the absolute best gear is at high end.
This will drive you insane, because every time you get a piece of new gear, you're going to be looking for completely different items. If you run a 10 man naxx, and it's your first go there...you might end up with 5 or 6 new pieces which would confuse you if you looked at their ratings in rawr.