The short answer is, “of course!” Smelling (the nose, tongue, and the vomeronasal organ) is a dog’s primary way to take in information about the world. Dogs are olfactory creatures, we are visual. Their sense of smell is part of what makes our dogs so amazing!
Asking a dog not to put her nose to the ground is like asking a human to walk around with a blindfold on. My only requirement on leash walking is that the leash be loose, not that the dog walk without sniffing, unless I specifically ask the dog to heel.
The long answer is, “it depends,” because there are different kinds of walks. On some walks, you might be in a hurry and on others, the purpose of the walk is to exercise the dog. Still others are just “potty walks,” where the goal is to get her to empty her bladder as quickly as possible. On the latter two types of walks, dogs should be allowed to sniff and learn about their world.
You can even use permission to sniff as a treat for your dog. Just ask for some sort of behavior that she knows, like heeling, and then at the end, tell her she can go sniff. So it would look like this: Say “Fifi, Heel” (Fifi then heels), followed by “Okay, Go Sniff!” Your dog won’t know what “Go sniff” means, at first, so go point out something that’s likely to have a good scent on it, and let your dog sniff around. When your dog looks like she’s just about done sniffing, say, “Let’s Go” and start walking again. That’s a simple way to teach her the meaning of “Let’s Go.” Since she was ready to go, it’s automatically reinforcing, but you can also praise her for coming with you.
When you’re on a walk where you’re in a hurry or trying to get your own exercise, you can simply say “Let’s Go” or your “Leave It” cue when she’s starting to sniff something. Make sure that you train those cues before trying to use them!
Here’s a fabulous video by Emily Larlham, aka Kikopup, on how to teach “Leave It.”