Some dogs don't really mind but if you are a stranger to the dog, you shouldn't go and reach out for the top of their head immediately. Normally I'd show them my hands first, then they'd usually sniff and/or lick it a bit, then I go wild.
If they like it, they'll relax and maybe even lie down. Otherwise they'll just back off and go back to doing their own thing.
A good thumb rub along the jaw muscles and gently rubbing the base of the back of the ears (this is much easier if the breed has cropped ears). Ryder makes a good point for those who do not know how to approach a dog.
My dog always liked it when I rubbed the base of his ears while petting and hugging him at the same time. Like Ryder said yiu shouldn't out right pet dogs that don't know you. Especially if you have a half-breed like I did.