Yep, like PPs said, when the next strap up is at or below her shoulder, then I move it up.
On that note, we have moved up the straps in our own car seat once (Chicco Keyfit).
But, we just got back from visiting family and renting a car with a car seat and were unable to move the straps in that to fit properly. It was the cheapest car set I have ever seen (no name branding on it anywhere--so no idea what make it is) and VERY difficult to adjust anything, even getting her in and out.
So I now understand that all car seats are different and some might be near impossible to adjust the straps.