What are the rules on parade rest and saluting when in combat or a field environment?

Believe it or not, the military runs as much on common sense as it does on regulation. The relaxation of saluting in combat areas probably came about because some officer got shot shortly after he was rendered a salute.
Why do we salute or stand at parade rest anyway? These courtesies are traditions for a couple of practical reasons; to recognize who is in charge and to stop the junior person from talking to provide the senior an opportunity to pass on instructions or greetings. The habit is established in garrison and extends to the field although the gestures are omitted for practical reasons.
There are many situations not clearly defined in the military. Just as we depend upon the junior person stepping forward to give direction in the absence of seniors, common sense is relied upon most of the time. Yes, there will be moments where the wrong decision is made, but it mostly works out.

Not sure if it needs to be done in a field environment because that's would still make that soldier stand out and the sniper would be interested thinking that parade rest behavior by the soldier indicates a leader present.

Let me ask a couple retired E-9s.
CSM Charles Hayden SGM Steve Wettstein

We used to have a slogan, "Train the way you fight". So if you would not do it in a fight, do not do it when training to fight.

The regulations do not specify that you do not have to salute in a combat situation or not to go to parade rest. That said the person to receive so said respect does have the right to order you not to while in combat or training environment. Most will ask you not to do this because it makes them the target of incoming fire. So if you do not really like your leader show him respect on the battlefield. I bet I catch all kinds of flak for that but some leaders get promoted to fast and get a big head and this is a good way to give them a wake up call. It works normally opens a dialogue with them and clears the air in a tactful way without much yelling and name calling.

Saluting is always allowed.
The common-sense reality is don't do things which may attract enemy fire, like standing at attention, saluting, returning a salute, or anything else eye-catching in an unfriendly neighborhood. Places where there isn't an enemy with line-of-sight view, you have much less reason for not rendering the traditional courtesies.

Think damnit- In the field, you are supposed to be training for combat in/under combat conditions- Most O's/NCO would discourage it. If you don't like your superiors, salute them or draw attention to them. Me, I would knock you on your azz.

SPC (Join to see) Lots of great perspectives already, but ill toss $.02:

The way I look at it is similar to LtCol Robert Quinter, who suggested that train as you fight is the golden rule. If in a combat situation you would not salute or go to parade rest, why do it during a fieldex?

For those looking to explain the "why?" to curious Soldiers, we train (especially battle drills) in a field environment to become proficient in repetitive or immediate tasks under operational stress. Conversely, in a garrison environment we salute and render customs and courtesies to reinforce the chain of command so that Soldiers know where the orders come from during operations. What we do in garrison, is the foundation for training in a field environment, which determines how we execute in a combat environment.

I would suggest that anyone defending the practice of saluting at forward operating bases in a combat zone are interpreting the regulations to suit their own perspective. But I have no seen anywhere that prohibits it.

I don't salute or go to parade rest in the field when I'm outside, and usually the saluting part is covered in a small brief at the start of the exercise. Field environments are typically relaxed for that due to what is done when forward. That said, snapping to when your leader is pissed is typically an auto response. You'll also see people do it when an LTC or full bird is giving a brief or talking with joe in the field. Ends up being a natural response to go to at least parade rest when speaking with rank even if going to attention is waved off.

Feel it out with your unit and your individual officers and NCO's, there's always some moron that wants to be sniper checked. Grant their wish.

The only training environment I have every been in which we were instructed to not follow courtesy was JRTC and NTC, and that was only in regards to saluting officers. I have never been in a situation where we have relaxed parade rest.