If you don't specify specific stashing for items the bot will just stash in the first available tab. It's more for organizing. The bot is more than capable of having say... 1 tab of 4 being for recipe items (specified in ComLib and AIF) and the other 3 tabs not even listed in ComLib. The bot will just deposit EVERYTHING not a recipe item then into those 3 tabs starting with the first until it's full and moving on.
Specific stashing is really for people who have more tabs. I have one for div cards, one for maps, one for quality flasks, one for quality gems, etc. Is it needed? Not really, AIF could do the GCP recipe without all of my gems being in one tab, but I like it organized.
You just have to remember other plugins may need access to those tabs. So if you're running MapBuddy for example, you would also need at least your tabs you put maps in and currency.
Honestly if you're only running 4 tabs (and not even a currency tab), just add all 4 to ComLib. You don't have to set specific stashing for anything it'll just look through those 4 for stuff. ComLib stash caching was originally created because when MapRunner came out and plugins like that, it would cycle through your whole stash. For people who have a TON of stash tabs every vendor task run was like a 5min ordeal just cycling through a bunch of tabs the bot didn't need to look in. Cached tabs cut that down.