Like BC said, you can do it however you like. You can have extra tabs in the drawer. MOst practice relading from one source primarily, so they are faster with that. Usually the on gun ammo. If you know it is empty and have the chance to switch out tabs do it. If not, load from wherever you put stuck the tabs to begin with.
Like I said, I don't use it or own it. I know that almost all the other systems either don't work well at all or are much more expensive. None are as flexible I know that some people who have a lot of time with a defensive shotgun use this system and like it.
The only downfall I can see is if you make your own and go cheap on the velcro or it starts to wear out and you don't replace it.