When I first started looking I looked at home depot and Lowes. The kitchen designer there told me there was a sale for 2 months on certain line on cabinets. At lowes there was actaully a sign posted showing your savings. If you asked the KD I bet they would tell you.

At the big boxes, all you have to do is walk in with another quote and Woah Nellie! Suddenly they're having a sale.

Go figure.

After viewing several lines and picking one that seems to suit your quality and price requirements, you create a relationship with a Kitchen Designer, that is, pay the deposit and hire one. "Shopping" is all well and good, but without a KD, you are going to be out in the cold on sales. Any good KD will represent several lines that potentially can give you "the look" that you want in your kitchen. Let them help you create the kitchen you want instead of trying to do all of this alone.

My cabinets are Medallion. I worked with a cabinet shop that carried 5 brands of cabinets. My designer would tell me things like "Let's wait until the beginning of the year to order cabinets because they're to lower the upcharge for painted cabinets". The charge went from 15% to 7%. Then when we were ready to order them, he said they were running another special (I forget what it was) that lowered the price even more). I never saw any of that information on Medallion's website, and I was a little surprised that he passed the 2nd savings on to me, because I had to buy the cabinets then, regardless of whether they were less or more than they were the month before.

I think I worked with a very honest show, based on the recommendation of my GC. We did price the cabinets at a local custom place and the custom place was a little less, but the time factor made me decide to go with Medallion.

There is no way to tell, you need to find a KD you trust. Two things to keep in mind: the salesman is likely more invested in making the sale than keeping his small percentage of the discount, commissions are not all that high.
Second the mfgs are biting at the bit to end the long run of discounts they have had for the last few years. Dealers whine when a promo is ending so they have kept them but business is up. I just received a schedule from one mfg who is phasing out a lot going standing promo over the next 6 months a little at a time. I expect some others to simply not renew promos some time within the year.