If you can easily remove them, it wouldn't be a bad idea. It's not really necessary though. When staining with iron nitrate I remove what I can, but the nose cap and inlays have to stay. If for example a butplate isn't removed, sometimes you get a band that doesn't darken up. I guess this is because it doesn't get heated enough with the butplate so close. I do generally get all the hardware back on the stock relatively early in the finishing process. If you wait too long, you risk parts not fitting, especially if sitting the stock in the sun to help finish curing.