Dominant Follicle?

I'm on Clomid round 2 and my doctor wants me to come in on my peak OPK to see if I have a dominant follicle. What's the reason behind this? How important is it? Hubby just had dental work done yesterday, and that ended up being expensive, and I know the ultrasound will be expensive too. So I'm wondering, would it be ok if I didn't get the ultrasound? What purpose does it even serve to know how big the follicle is?

Please no rude comments.

If we don't get pregnant this Clomid cycle, we're taking a break from TTC for a while