It's absolutely fine to stretch without using a taper. It's simply easier with a taper device. However a taper shouldn't be used once you pass 10mm (00g) anyway.
My only advice would be to not used a flared plug or tunnel and make sure your ears have fully healed before you attempt to size up. You'll also want to use surgical stainless steel or glass instead of other materials. Your best bet would be a no flare tunnel that has 2 o-rings to keep it in place.
You could also use the same tunnel with the taping method if you wish to carry on stretching in the future.
Hope this helps!