This all depends on what style of 7-day candle you have.
It it is as glass encased 7-day candle, placing the herbs to the edge of the candle, against the glass is a good way to limit how easily they catch alight and reduces the chance of them putting the flame out. If you are using a glass candle it is also good to use only a very small amount of herbs, too many and the candle will repeatedly go out. After the first hour or so, any herbs on the top should have sunk into the wax and no-longer pose a risk.
If it is a freestanding 7-day candle, I have found the best way is to either load the candle into the bottom and re-seal the hole, or to dress the candle with oil, finely grind your herbs and press them into the oil. Doing it this way allows the herbs to be over the entire candle in a small quantity, they won't fall off but also don't post the same risk as having them at the top of the freestanding candle.