it depens of the capabilities requires by the API as well as which channels you are thinking of distributing the signed application.
Basically if the APIs you are using are only requiring capabilities that can be self-signed, then you could use self-signing and distribute the application as you wish. And if you want to use Nokia Store for distributing, you could ask for UID from the store and use the store signing as well.
Then if the APIs in your application are requiring capabilties that can not be self-sined, bbut would not require certified signing, you could use the Ovi signing again, provided that you only use Ovi for distribution channel.
if you want to use other channels for distribution, then you need to get Publicer ID and use Symbian signed portal for signing.
And if any API used in your application required capabilities that are requiring certified signing, then you need to use symbian signed portal for signing, and you do need to have publicer ID as well.