I think this depends very much on your social circle and social goals. Wearing clothing with slogans on it is a high variance strategy: high attractiveness to a few people, low or even negative attractiveness to others. Wearing slogan-less clothing is more low variance; probably no one will object, but likely none of your responses will be as positive as the maximum positive response from wearing a T-shirt with a slogan on it. Both strategies can be useful, depending on what you are trying to accomplish.
Personally, I wear shirts with nerdy slogans on them, and anecdotally have had several positive interactions with people who came up to me to say "I like your shirt." (And I doubt I've lost much by turning people off.)
Also, I'm unconvinced that, in a casual context, wearing a shirt with a slogan on it is as negative as you suggest. I see people wearing shirts with slogans I don't get all the time, and I think I just ignore them, or occasionally ask what they mean (which rarely gets me very far conversation-wise, but doesn't cause me to dislike the person).
On the other hand, if you're trying to project an aura of Serious Grownup, it's probably a bad idea.
EDIT: Unless you're talking about shirts with controversial slogans, I suppose. That's even more high-variance, but again, in some contexts could still be a good idea. (I was thinking of things like "Engineering: It's like math, but louder.")
Wearing slogan-less clothing is more low variance; probably no one will object, but likely none of your responses will be as positive as the maximum positive response from wearing a T-shirt with a slogan on it.
Depends what you're wearing instead. T-shirts with cute slogans on them are nowhere near the only way to make an impression with your clothes; in fact, I'd consider them a pretty cheap and lazy way to send a message as such things go.
You're probably right if I'm to take a plain T-shirt and Levi 501s as the implied alternative, though.
What are your best short witticisms, suitable for use on a t-shirt, bumper sticker, or similar location? Ideally something that might make someone reading it think, or get curious enough to ask about it. Simple in-group identification is fine too, though.
For context, therufs is spending today at the NC Maker Faire making t-shirts, and asked me for suggestions this morning. As I was still mostly asleep, I wasn't very helpful.