The only one that really seemed out of place to me, was when she was clearly trying not to laugh after saying "Susie!", in the episode with the rat babies.

Haha I totally believe that! Every time I watch that part I think "I'm sure Jennifer is grateful they let her laugh here-it must've not taken a lot of acting on her part!" Unlike Matthew and Courtney who had to keep pretty straight faces.

Everyone was supposed to hate his playing.There's the blooper on it where everyone is laughing the second she starts her singing. But in the actual episode, Courteney, David, and Mathew kept straight face, but Jen didn't. We see her trying to hold back a laugh while David is playing.
And even Lisa still laughs just a bit at the end.
In the DVD with commentary. David says Jennifer couldn't hold her laughter. And they mentioned cutting around the actors laughing. So official sources have stated Jen was not supposed to be laughing there, which is why she's covering her mouth and hiding her face. But it was probably the best they were gonna get. So it can work as Rachel laughing at Phoebe, but that wasn't the intent. lol

If you're gonna pretend to cut her hair, at least put some scissors in your hand!

It doesn't bother me too much, but I would still say that example is different. Rachel was obviously supposed to be laughing at Ross.

What I find funny about the scene with Monica asking about keeping a second copy, 100 yards away. Phoebe replies she didn't do that because she's normal. Lmao
When has Phoebe ever been normal?
In any case, I agree. Best to not overthink it. The characters can laugh. Look at how Jen couldn't keep a straight face when Phoebe starts singing to Ross' bagpipe-playing.
And Matt LeBlanc is guilty as well sometimes.
http://ic.pics.livejournal.com/silverfang2/29185537/934407/934407_original.png

If you're gonna pretend to cut her hair, at least put some scissors in your hand!

Yeah but there are other ones too. Like when Phoebe's cookie recipe burns up and Monica asks her why she didn't keep a second copy in a fireproof box 100 yards away from the original. You might be able to argue that too, but I know there are other ones where it wouldn't make sense that Phoebe would be cracking a smile.

I don't overthink that too much, because in real life, people laugh. At their own jokes even. Phoebe, the character, could have just not been able to say that with a straight face, as it was quite obvious that the information was not new to her, when she said that.

There are a couple of moments that I've noticed MP break character and laugh/smile.
Maybe that bloody dagger will lead us to the murder weapon.

Yeah I also notice loads of those too, i also saw Matthew Perry do it once for like a split moment but im darned if i can remember what episode!!
cArNiVaLs oF fAyGo

I've noticed more times when she's about to burst out laughing, like in TOW Rachel tells Ross, when she shouts out, "that is brand new information!" You can see her smiling. I know there are like 2 or 3 other ones but I can't remember specifically when they happen off hand.

I'm pretty sure she's just looking around because she's flustered.