Colleague and criticism

Over one lunchtime, me and two of my clinical lab groupmates, E and Z, had an interesting albeit short conservation about criticism. As we know that we all will be working together, and seeing each other almost 24/7 next year. We will really get to know each other through and through, and it will be similar to having intensive orgies relationships with an expiry date as of Dec 2012. There are bound to be some degree of interpersonal conflicts, if we don't communicate properly. Our group consisted of six people, equal number of guys and girls, but only one straight guy, lol. L is my lab partner, and we often have cafe hunts together. She is also my philosophy conversee. Z is the poor straight guy, who frequently answers a lot of my questions regarding treatment. Before I digress any further, how can we provide criticisms due to good intentions, without upsetting someone?

Halitosis (bad breath)

Me: Let's start with bad breath. If you start talking to someone and notice the smell, how would you deal with it?
E: Hmm.. that's hard. But if I know that person well enough, I would tell that person outright though not out loud. Otherwise, I will hand out some chewy or mints.
Z: What if you don't have any of those with you? What if that person is a lecturer/teaching clinician, whom we will be spending a lot of time with?
Me: I guess for that case the prognosis is hopeless. Well... better start training holding your breath then?
E: Oh.... for your information, please let me know if I have a bad breath. I don't wanna be talking to clients while they try not to breath. Hey... GVP, you're quite lucky then. Don't you get use to all the bad smell, since you spend so much time in the PM room?
Me: You're right. So far I can even put my face near rotten horse guts, and it seems that I have instinctively learnt to shut off my nose, but I can still smell farts and bad breaths somehow. Another thing, this year I have morped into a little grumpy beeping bomb. I know that I swear too much, but I can't help for now.
Z: That's alright. We are quite stressed right now, so everyone swears and used to it. Remember the surgery lectures, how many times did we hear vagina, penis, testes?
E: There were so many vaginas, penises and testes. That was more than enough.
Me: One thing that we won't have enough of next year rotations will be sleep. I will definitely get moody, and swear a tad more than usually. So tell me to f!@# off and get some coffee, if I'm acting like a dick.
Z: Only if you promise, you won't treat me as your PM customer after I tell you so.

In the end, we also talked about dandruff, dress code, low jeans exposing ass crack, etc.  We all agree that we prefer honesty, and don't mind being told off if there is something odd. Feeling upset is inevitable at first, but as long as there is a good motive behind, everything will be alright. One point that we took, before rushing to our prac class was that all of us are not perfect, but at the very least we strive to improve. Being optimistic as I am, I often think that if someone criticise me, it's only because they care about me. What is your approach to this?