It's normal, CPUs instantly draw more power when they need it so instant increases and decreases of temperature are normal there is no grace period of it slowly increasing(there are factors that can cause this like water cooling takes longer to reach equilibrium so temps will slowly increase for a time but generally this isn't the case), and if you aren't overclocking you will be fine with the stock cooler. So it just comes down to if you're ok with these temps, If not then go for one, if you're fine then don't worry about it.