These are the minimum meta tags required for license reporting. If you edit the metadata in Airtime it saves the values back to the files so you can download them once edited and they will always be right.
Title, Creator (Artist), Album, Genre, Year
Other meta tags are very useful, ISRC Number for example as this unique code tells the licensing authorities who to pay any royalties to. If this is missing there is a chance that the performing artist will never get any royalties because it could be claimed that they didn't know who to pay it to.

There are several places where you can lookup an ISRC code for an artist/track combination, personally I use this one most of the time as it is SoundExchange.