First, you need to consider the temperature. If you're in a cold area and your dog’s coat doesn't dry fully before bedtime, your dog could end up getting too cold. Secondly, some dogs have a habit of getting rather energetic after a bath, which might not be ideal before bedtime if you want them to settle down and sleep. Thirdly, if bathing stresses your dog out, it might be better to do it earlier in the day to give them time to relax before sleep.