Where exactly was the scar on Harry's forehead?

If the cover art counts, the cover art in the first book (the one I had read) showed it slightly off centre, but it's different in different versions

This page has a list of cover arts.

This seems to give a hint about where Rowling wanted the scar (for the first movie at least):

Rowling discussed the facial blemish at length with director Chris Columbus, requesting that it be "razor sharp, just off center".

Link

I don’t think the location is ever specified, beyond it being on his forehead.

When it’s first introduced in Philosopher’s Stone, Harry is just a baby, and all we know is that it’s on his forehead, behind some hair.

Under a tuft of jet-black hair over his forehead they could see a curiously shaped cut, like a bolt of lightning.

It gets a few other mentions throughout the book, where it’s consistently described as “on his forehead”, but with no further detail. For example:

The only thing Harry liked about his own appearance was a very thin scar on his forehead which was shaped like a bolt of lightning. […]

Mr Ollivander touched the lightning scar on Harry’s forehead with a long, white finger.

Most of the later books make reference to it near the beginning, to refresh the reader’s memory. None that I can find refer to the position on his forehead. Here are a few examples:

In Chamber of Secrets:

He wore round glasses, and on his forehead was a thin, lightning-shaped scar.

In Goblet of Fire, a chapter called “The Scar”:

The old scar on his forehead, which was shaped like a bolt of lightning

Order of the Phoenix:

the old scar on his forehead prickled uncomfortably

Or Half-Blood Prince:

“Oho,” he [Slughorn] said, his large round eyes flying to Harry’s forehead and the lightning-shaped scar it bore. “Oho!”

All the way to the end of Deathly Hallows:

He lowered his hand absentmindedly and touched the lightning scar on his forehead.

None of these quotes make any reference to the location of the scar, and there are others like them throughout the book. This isn’t a proof that it isn’t described in the books somewhere, but most descriptions seem to omit it, so I don’t think it’s there.

(Which is not to say JKR did not have an idea of where it was – as the quote in user13267’s answer shows, she had some thoughts on it – merely that it doesn’t show up in the books.)

Rowling herself didn't even know the actual answer, she picked these images at first because she had this feeling that it will be the Harry Potter she's searching for. But the other books came out after the first movie, and since the first movie says it's above the right eye, she had to repeat how Harry Potter is supposed to look like in readers mind.

Over his right eye

Rain pounded on the windowpanes as they strode along the empty corridors back to Gryffindor Tower. Harry felt as though his first day had lasted a week, but he still had a mountain of homework to do before bed. A dull pounding pain was developing over his right eye. He glanced out of a rain-washed window at the dark grounds as they turned into the Fat Lady’s corridor. There was still no light in Hagrid’s cabin.

_{Harry Potter and the Order of the Phoenix - Chapter 13}

Most of Rowling's eight illustrations of Harry that we have do not show his scar at all. The two that do show it end over his right eye.

_{1999 BBC Children in Need character sketch (full size image)}

_{Page 18 of the 2013 English PEN annotated Philosopher's Stone (see full size image)}