So do you have to reapply thermal paste each time you take off the heat sink from the CPU? 

Yes, because thermal paste is the medium for heat transfer between the two surfaces. If it's not there evenly, you don't get proper heat transfer.

It doesn't matter what heatsink you are using... stock aftermarket. You don't want to remove it then just put it back on. Why do you keep removing it anyway? When you remove it the paste as you can see is pretty dry some will even pull up, you won't get good coverage anymore and will most likely have spots with nothing and or air pockets.
 
Also anything you buy will be better than stock most likely and better than re using whats on there for sure. I wouldn't really worry to much about which kind your going to use if your just using a stock heatsink... doesn't sound like your going to be OCing. I just use rubbing alcohol to clean off the old with some paper towels or q-tips.

He had to take his CPU heatsink off because he had the post code 55 error. His CPU heatsink was on too tight so he had to re adjust it. Plus Godrillax, make sure you get that one heatsink I recommend you once you deiced to get one. Like I said you wont regret it once you start to do your OCing with your build.
 
If you want the best thermal paste thats out there then get the Indigo Xtreme paste. It has the highest thermal conductivity than any paste out there which is 20 W/mK. Of course this come with a hefty price tag of $10 for two applies but you pay for what you get.
http://indigo-xtreme.com/
 
Now if you want the second best paste and which is  more affordable and comes with more applies, get the Phobya HeGrease Extreme Thermal Paste. This paste is ALOT better than that cheap crap such as Arctic Silver 5 because it has a higher thermal conductivity of 8.5 W / mK and it wont short anything out if any paste seeps out of your heatsink on to your board by you making the mistake of applying too much paste.
http://www.amazon.com/HeG...Compound/dp/B0067M0ZAU
 
 
Check out this chart to see whats the best and whats not. As you can see Indigo Xtreme is at the top because it is the best.

ive done it before without reaaplying. Thats because the paste was still fresh though. Only been a day since i applied it. So when i removed the heatsink i just used my finger to spread it out evenly again then remounted the heatsink.

I always clean the thermal paste off completely and reapply fresh paste every time I break the contact between CPU and heatsink.
 
For cleaning, this is my favorite. Works WAY better than rubbing alcohol: http://www.newegg.com/Pro...010&Tpk=arcticlean
 
I personally use Shin Etsu and Prolimatech thermal paste. They aren't over the top expensive and perform extremely well!
 
http://www.newegg.com/Pro...x?Item=N82E16835242027
 
http://www.frozencpu.com/...Compound_-_5_Gram.html

if paste is fresh (and still soft, blah blah), then I don't clean nor re-apply.
if it gots hardened already then you have to clean & re-apply because it's nearly impossible to fit the cooling block the very same way (to the micron) so all matches up without creating big air pockets.