It's very unlikely to get anywhere near the temperature where thermal-throttling will kick in, and if it does, it's totally harmless. There are only very specific circumstances where thermal-throttling might lead to you wanting to fit heatsinks, and one of those is if you're heavily overclocked and running any particularly heavy-duty 3D games where it may lead to a lumpy framerate.
I have a thermally-controlled fan for this specific reason, but the vast majority of Pi users will never need additional cooling.
Pi2B Mini-PC/Media Centre: ARM=1GHz (+3), Core=500MHz, v3d=500MHz, h264=333MHz, RAM=DDR2-1200 (+6/+4/+4+schmoo). Sandisk Ultra HC-I 32GB microSD card on '50=100' OCed slot (42MB/s read) running Raspbian/KODI16, Seagate 3.5" 1.5TB HDD mass storage.