CD sectors are replaced by 0x55 because most common tools do that by default, there was no reason for us to switch to any other pattern. Other than that, using 0x00 for CD would probably be unreasonable in therms of detection of replaced sectors if needed.
DVDs barely have those protection mechanisms and i am not sure if any tool other that isobuster gives you an option to replace unreadable sectors by a byte pattern of your choice. 0x00 sounds okay for DVD sector replacement, at least i feel so. And so do common xbox dumping tools as well.
If i remember correctly SS areas of Xbox discs aren't stable, each ss range has different amount of readable sectors in the beginning and in the end, and if i recall it right the amount of readable sectors differs also between drives.