This is not a one size fits all question. It depends on the dog and the handler and how there relationship is. In obedience training an instructor gives advice that is suitable for the majority of the class. They will give individual tips as necessary and as time in the class permits. However, it is the handlers responsibility to have control of their dog at all times!

So I believe the answer to this question of letting your dog walk in front of you during walks depends on your dog and it's relationship with you. If you have control of your dog at all times and the dog respects you as it's leader then letting it walk ahead of you could be a pleasant experience for the dog so why not let it enjoy his walk too. Just never settle for pulling or an uncontrollable dog.

On the other hand, if your dog is out of control on walks with constant pulling and doesn't respect you as the leader (a whole different discussion topic), then you need to make walks more structured and a time for learning along with the exercise it is getting. In this situation you may need to find a way of releasing the dogs pent up energy before the walk so it will be a more beneficial learning experience instead of a pulling contest. Some dogs have so much energy that they just can't focus on heeling unless some of it is expelled first.

The bottom line.... only you know if walking ahead of you on a walk is bad or not for your dog. I personally let both of mine explore as long as they keep my pace and heel when told to for their safety (ie.. a car coming). After all, I'm walking them for their benefit as well as mine.