I make my own DIY juices and definitely shake the hell outta everything, first I make a solution of the nic/vg/pg and shake that up, then make a solution of the flavoring and shake that up, then mix it together and shake that up, let it sit for a bit, shake it up some more, let it sit a day, shake it up some more, etc lol. Playing around with different flavoring and pg/vg/nic ratios you can really see the different separation in the juices while it's all coming together. When I first make it I'll give it a good 30 second shake, inspect it, shake it more if needed, let it sit, then come back and inspect it, just to make sure it's all bonding right. I'm not sure what method vendors use but I'm pretty sure it's something similar so I would definitely say shake it up before use. I always do (bout 3-4 seconds before filling anything).