Hi TS, when u talk about photography, you talk about lights. So by using the Av mode for Panorama u will face problem when the area are thought to be bright or dark by our camera. Just like if you take photos of snow with AV or TV mode, the snow turn grey and not white. That's just an example.

Since we are stitching the this Panorama picture, we canot do that unless u are sure that the Speed and apeture is the same for all the picture (rare case). On top of that, for me ISO should also set to Manual too. But before that u have to measure the lightings for the settings for Panorama. Example: 1/120s, F9.0 ISO400.

So, Is manual important? Yes. And also it's very useful if u use with flashes.