Think of it this way. Outbox are how you get what you want to say out to the rest of Nostr. That being the case, you will want to use relays that anyone can read from. Now, that's the case with MOST relays, so you have a lot of options here.
Inbox relays, however, will determine who is able to get what they have to say, particularly in comments and replies, in front of you. These relays are really useful for minimising reply spam if you use them correctly by only adding relays that people you care to hear from can write to. The best options here are Web-of-Trust relays of one form or another, or paid relays, if you really want to lock it down, or a combination of both.