Time steeps all things beautifully... given enough time. So yes, sitting on those store made juices, or your own, certainly matures them. What we are helping you with here is how to best speed that process along, or at least kick start it a little.
Ill take dannv's post a bit further tho if he doesnt mind and amend it a little. I find four ways to steep:
1) Time does it all on its own... but patience is a virtue and checking a juice eveey few days to see if it is mature YET is just how it goes. As you find your favorite juices... you'll know almost to the day of steeping time when it will be prime and ready.
2) Hot water. This to me is boil or microwave some hot water, drop in your bottle and when the water cools, redo it... I find this very consuming and VERY ineffective. Yes it helps some, but not alot.
3) CONSTANT Heat... in a hot water bath. Now thats the method in my experience that works best. Steady heat is more effective at producing a good steep. I dunno why that crock pot at 125° works so darn well... but it just does, and I believe the steady heat/temperature is the reason why.
4) Ultrasonic. People swear by it but Im not sold that it is more effective. I can shake things by hand. Heat and ultrasonic well thats exactly same as 3 above really. Yeah, okay we can get all Bill Nye science guy here and talk about microscopic blah blah that may/may not happen with ultrasonic steeping but the investment in an ultrasonic cleaner that has heat just, imo, isnt worth the expense unless you wear alot of jewlrey or use cartos alot... it does work well on cleaning attys and cartos.
See, Heat does more for infusing flavor than any other method. Just like cooking. Its all about the flavor molecules. Heat infuses... shaking... just shakes. It does nothing. Anyone can shake/mix pasta and sauce. But add your pasta to sauce, with heat in a pan, and the sauce coats the pasta and infuses it with flavor. Much better than just thowing sauce on top of spagetti, yuk. Heat the sauce and pasta togerher in another pan with a touch of the pasta water for a final cook, and its a whole new world of flavor.
Fill your crock pot with water and set it on its Low setting for about 30mins. Check again 10 mins later to see if the temp has remained constant. Use a thermometer to check the temp. If its above 125° on Low, redo and then check the warm setting for temp. (Best guess, on your crock "warm" is likely 125° or close to it) but start on your low setting just in case. You can always adjust up or down once you know where the middle takes you.
During that three hour steep... if you are using alc base flavorings keep the tops off for the first half hour. Then caps on for the remainder of the time. Any pg, or, vg base flavorings caps on the whole time. Shake the bottle by hand every 30 mins or so.
I only steep with caps off IF the flavorings are made with alcohol, or if its an alcohol based flavoring like rum flavor for example. Or if its very floral or perfumey. smelling.