Now my question is, how should I deal with these pacnew files? should I always remove them, always replace them, always read them and decide?

Always read them and decide what to do. However, what to do usually isn't too remove or replace them, but to update them with the changes instead. Most text editors have a way of looking at a diff of two files. This will highlight the differences and you can decide based on the individual changes (maybe it's something you purposefully changed, maybe it's a change to the default). If you use vim, vimdiff will do this.

I'd rather not read these things everyday, it's a bit boring, so I hope there's a better solution. How do you deal with these?

After you make your decisions on what to do, delete the pacnew, otherwise you'll keep getting messages about it. They don't get updated all that often (except mirrorlist, I usually just delete that and run reflector every once in a while).