It depends what you're putting in them. Some favors don't really need it, but a lot do. Alcohol-based flavorings, for example, really need it.
As far as the best method.. well, that's open to debate. What I usually do personally is leave the cap off overnight after a good shake to let the alcohol evaporate. Then (unless I still smell alcohol) I'll recap them and shake once or twice a day until they've steeped long enough.
How long that is will vary depending on the
juice.