Hello. This movement has a power reserve of 48 hours. I've seen everywhere that we should wind a mechanical watch everyday at the same hour. But when a watch has a two days power reserve, is there any special reason to keep that rule? Thanks in advance for your answers.

 

If you plan on wearing the watch everyday I would try and wind it at the same time every morning for the simple reason that I would remember to do so.
It also makes sense to wind it every day to maintain its energy and accuracy. The accuracy of the watch will diminish towards the end of the power reserve due to the mainspring not being fully wound. I would assume that a manual wind watch is more accurate when fully wound as compared to the end of the power reserve.

 

The danger in winding your Omega after a 48-hour run is that, if you get busy and forget to wind it, it could run down on you. Not normally critical, but it could cause you a significant problem if it happened on the wrong day! And, as has been suggested, the rate could well deteriorate toward the end of the second day.