+1 on kaynin's point. You will definitely get people who claim to "know the eula inside and out", be experts at "what is and isn't automation", heard rumors from friends about blizzard banning multiboxing in the next patch, or claim how it's only half legal (as in it's ok for PVE but you can't run BGs.)

When I was in a raiding guild, I suggested to my guildies that I was thinking about 5 boxing. My GM told me he read on official PTR forums that keyboard replicating software was going to be banned in patch 2.4, and that I should "have fun with it while it lasts." I asked for a link, he couldn't find it and didn't pursue the conversation.

So, yes. Even people you trust and have raided with for a while will have their own opinions, and may try and discourage you if they disagree with the concept.

Don't be discouraged. All you have to do is read the [extremely] long list of blue posts linked on Vyndree's blog and you'll feel better. Short and sweet, you can always tell people to "search for multiboxing on wowwiki, it is absolutely legal." Referencing a trusted and known source like wowwiki usually simmers them down.