Surname reservation is.... the reservation of a surname, in order to prevent others to use it while ensuring you can.
Surnames will come two ways, first one is non CoE lore related surname or lightly inspired ones. Those are the real surnames you want to reserve because those are the one at risk to be used by someone else. The second family of surnames are going to be the ones heavily inspired by CoE lore, among them the generated ones.
Sure some generated surnames might please you more than others, but the chances someone else actually get the same generated name only depends on the generator, if it is any good and i have high hopes it will be, it should generate quasi-unique names, limiting the chances that the same generation happen again.
So if you do not want to secure a non or low CoE lore related name, no need to wait, you generate a few surnames you like for each tribe, and later, when you'll actually select your tribe you'll have a few words to select from with really low chances someone else took them. And in the very unlikely event all of them have been taken, no big deal, you only need to generate more till you find a new one that pleases you.