I don't have a big issue with those who TP with Iron - it's not my piece of pie, but I do think it's a legit way to play.
Agreed, I have no issue with TP with Iron. TPing with lv22 Bullets, no way. But Iron is OK with me - Steel is a big step up in cost, and for those who can't synth them yourself or have an on-call synth friend, sometimes the AH doesn't provide much help with Steel. I've seen periods of time where for days there are ZERO stacks/pouches of Steel Bullets on the AH, and when one enterprising crafter tries to put a few up they demand extortionist prices.
That's why I try to keep a bunch of ammo in reserve (I usually own 12-24 stacks of Steel at any given time, and I get nervous when I break into my last stack of 12 pouches), but I understand that sometimes you just want to go out and party and you realize you're low on ammo or funds. Your party is probably more interested in having you come and use Iron than having you pass up the party altogether if your bank account or ammo reserves are suffering at the moment.
I think reasonable rules for ammo choice:
1. For WS (and QD, which doesn't even use the bullet) you need to use Steel if you're 66+. No exceptions.
2. For important events, missions, etc. - use Steel.
3. For pickup exp/merit parties, I've got no problem with TPing with Iron. Certainly Steel is even better, but I wouldn't call it "gimp" to use Iron. Just like I wouldn't call a 66MNK using a Scorpion Harness "gimp" because they aren't using an SH+1. Is the HQ better? Absolutely. Are you an embarassment to your job for using the still solid NQ? No way. Now if the MNK was DDing in AF body (or a 66+ COR was fulltiming regular Bullets), that IS gimp.
4. If you're TPing with a multihit weapon, I see no reason at all to use Iron. Your ammo is presumably all (or at least nearly all) going to WS and your whole strategy is revolving around gaining TP quickly to skew your damage over to WS, so you better be using the good stuff for ammo.
For the switch from Bullets to Iron Bullets, it's even more simple:
1. Are you lv.55+? Use Iron.
2. Only exception that I would excuse is full-time pulling at a fast camp. If all you're doing is pull/Light Shot/buff, Bullet is fine. The small amount of extra damage from one /ra using Iron/Steel isn't going to substantially change the fact that the mobs are dying really fast anyway from your DDs. For the occasional WS and your /RA if you have enough time to fire off an extra shot, macro in the best ammo available.
There are always those uncommon situational uses too. Someone in a skill-up party re-capping Marksmanship after leveling COR in sync parties, for example. I recently went through a couple stacks of normal Bullets breaking weapon latents for myself and a friend (to get Detonator for me, then using Det to help him break Destroyers on MNK). No point in spending extra gil there, and mobs staying alive a little longer can actually be helpful in speeding the process up.