When to see the pediatrician?

My sister called me that her son (6 months) is running a fever of 102.6, and of course it would be a Saturday night. She was asking me when she should take him to the pediatrican, and this has always been something that I'm not entirely certain. My general rule of thumb has always been to call the doctor's office if Chloe's temp goes to 103 and take her in if she starts to act lethargic or out of her norm. (I think this is what the nurse told me, but it's been a few months. *knock on wood*)

When do you take your little ones to see the doctor if he/she is sick?