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If you run it in the background then your PC will slow down and your other work also may slow down. I also don't know how it will affect my PC function in the long run for utilizing CPU power always at higher side. I don't think it is worth at least for myself and don't want to risk my PC to earn these small amounts of bitcoins.
I did quite a bit of testing on this and setting the mining at 20% had no noticeable impact on anything I use on a day to day basis. It really is quite odd that anyone could be worried about the impact of using a processor to do processing. As long as it is properly cooled and the safety cutout systems are all working to prevent overheating running at 100% is perfectly safe. Running at 20~30% is just normal usage.