I've never used numbing cream for any of my tattoos so I can't speak to that but I have used Bactine (it's an antiseptic / numbing agent that's meant for first-aid, not just tattoos) thrice - for my lower back, the back of my knee and my chest. The "wound" has to be open - you can't just spritz it on fresh skin and then start tattooing - but after it's applied it will numb the area for a little bit and help reduce swelling. I certainly found it to be helpful but artists are also careful not to overuse it.
Tattoo artists' opinions of numbing agents vary on an artist-by-artist basis but everyone I've met tends to not have a problem using them; bear in mind that it also depends on what part of your body you're getting tattooed and how long you've been going for. If you're getting something that isn't anywhere particularly tricky (like your arm), are only fifteen minutes in and ask for it, chances are it won't fly.