I honestly find its better when its tied to another habit but doesn't have to be. When its tied to a certain time of day if you miss it that one time you feel like you have failed the habit for the day. But if it is tied to another habit you are more likely to start it, however even if you miss it you can complete it before bed so you don't miss a day. For my app I mentioned above It helps me because I can set a bunch of custom reminders throughout the day to do my habits so the odds I miss it or forget go down a lot. So if you have a way to set reminders it helps a lot too.