Only the ones you're actually interested into trade.
If you plan to federate through many altcurrencies is probably because you plan to provide a service; if you do plan to offer a service it's perfectly reasonable to you can provide at least 1TB storage as well to use in your network.
The common use is supposed to be sync between three blockchians: the two altcoins and the federative one.
For example, if you're a dogecoin user... you may be interested into trade just between bitcoin and dogecoin because bitcoin serves better (atm) as value storage move (widely supported).