Frankly, I have a lot more respect for companies following ISO9K2K than those who followed the 1994 version.
However, my philosophy vis a vis registered ISO9k2k shops is "Trust everybody, but count the cards!"
I don't buy ANY custom work without a personal visit by some qualified person from our organization to the work site and subsequent approval. ISO9k2k registration just gives us a [usually] easily followed roadmap to the QMS.
Standard item purchases of components (fasteners, cable, connectors, etc.) may entail correspondence to ensure we can have complete traceability and we will not receive mixed lots of goods.
(i.e. if we order one thousand bolts and they come from two separate lots, we want each lot packaged and identified separately, not mixed in one package. For example, when we manufacture an assembly, we ensure only bolts from the same lot are included in an individual assembly. If that's impossible, we maintain a chart of which bolts from which lot went where on the individual assembly and keep the chart with the Quality records. We're obsessive about this because we adhere to FAA-PMA traceability expectations for products which go into commercial aircraft.)
Summary of ISO9k2k registered vs unregistered:
I generally find more consistency (products, documentation, service) in companies which are registered or self-declared. I attribute this to general ignorance of good quality system practices by the others. I have found absolutely no difference in any material way between formally registered and self-declared organizations. Almost always, a company that's self-declared is not bluffing. I still do a lot of business with non-ISO9k2k organizations as long as I am satisfied product quality, service, and documentation associated with my products will meet my requirements.
(when I have a piece of sheet aluminum cut to my specs, there are few hidden factors which cannot be covered with some foresight - material cert, plating cert, sample of the trim material to check for hardness, tensile strength, chemical composition, etc. With those details negotiated before the product is made, I've never been disappointed - usually the supplier is happy to have such a detailed list. We are obsessive on traceability, but we also make sure we don't over-engineer anything by using 3 decimal tolerances when one decimal will serve or fussing about 1 or 2 degrees on a corner radius when no fit or function is involved.)