My family have trained gundogs for four generations, and our experience can be summarised as follows:
Some dogs are born with permanent hard mouths, and these can't be changed.
Some dogs are born with permanent soft mouths and will always have soft mouths regardless of what happens.
The remainder can be trained (with varying degrees of effort needed) to have soft mouths but can require retraining if an event or new situation has caused them to over squeeze.
As a rough rule of thumb, our experience is that the more intelligent and calmer the dog, the less likely it is to be (or become) hard mouthed. Since we are firm believers that increasing training (in variety and frequency) increases intelligence and that dogs shouldn't be rushed into the field, it is relatively easy to turn out a soft mouthed dog.
However, in the past, we have had dogs returned for retraining when they have become hard mouthed and almost invariably, is down to the owner putting the dog to a situation where it has had to either bite something hard (usually foxes or other dogs) or in a new situation where the dog has been left to work out what to do by itself (such as getting into rearing pens).
To answer your question, if your dog has a good mouth then I wouldn't worry too much about it. Give him plenty of supervision and instruction in new situations so that he is confident rather than nervous, and consistently reward appropriate behaviour and correct inappropriate. Keep the variety of the work up so that he doesn't start to get over confident or lazy, and I personally would avoid putting him in situations where he has to bite/squeeze hard - if it happens inadvertently then so be it, but my preference would be to hold at bay and bark until you come up to complete matters.
I don't play tugging and pulling games because I don't like the attitude that can develop in some dogs, but I don't believe that it encourages dogs to be hard mouthed - their mouths are their primary means of experiencing by touch and they are very good at discriminating between the right amount of force for different activities.
Most importantly of all is to enjoy yourself and your dog - if you are having a good time, then he will and therefore be much less likely to over-react.