Since I have been getting reports lately of parties grouping up that shouldn't be... I'm posting this ruling.
Your character has an alignment. In most cases, you chose your character's alignment. In most cases you play your character's alignment.
Partying with a person who is against your alignment is frowned upon unless there is very good IC reason for your character to do so.
Now, we understand that you may not know that the person you are with is good or evil, if you have never met them or not seen the evil in them or whatever.
HOWEVER, if you KNOWINGLY party up with someone who is of an opposite alignment to you, and there is no viable IC reason for that party to exist, and a DM sees this, you can expect the following.
1. A Tell from the DM asking why you are partied up with such and such.
2. Possibly a request from the DM telling you to break party and get into character.
3. Your character may possibly receive a hit to your alignment as a result of your knowingly associating with a person of that opposite alignment. It may not be a big hit, but it will be a hit none-the-less if the decision is made to do so.
Now I understand that some characters may be trying to redeem themselves, and that is fine. We will acknowledge that as a viable IC reason for the party up, etc.
Basically this goes back to the Metagaming ruling. PLAY YOUR CHARACTER, NOT THE PLAYER.
Expect consequences for your actions if you choose to hang around with, party up with, dungeon crawl with, people of opposite alignments who your character has no business being with.
IE: The Uber Evil Drow partied up with the Paladin. Or the known baby killer being partied up with someone who strives towards caring for all orphans. ETC.
Respect is Earned! Fear is Demanded!