One of the most striking things about McMaster-Carr's website is the nearly nonexistent top level navigation. I thought I'd compare it with some similar sites for context.

McMaster-Carr

McMaster's top level navigation has six primary items:

Search
Contact
Bookmarks
Order History
Build Order
Log in/Authenticated "my account" dropdown

The Search bar has a dropdown that autopopulates results as the user enters text. The Contact, Bookmarks, Order History and Build Order links are just that - links. No dropdown/flyout, nothing.

The unauthenticated site has a small "Log in" dropdown at the top right that takes an email address. The authenticated site's account link drops down to show a "Log out" and a "Settings" link.

MSC

MSC has too much top level navigation to describe in detail here, but a few takeaways:

Two home links (one is the logo).
They have a "Special Offers" flyout that has 6 items in it.
They have a virtual paper catalog link with a large logo next to it.
They allow you to arrange product categories by four variables.
They only show half of their categories by default; users need to click to show the other half (or, as a second option, "View All Categories").
They have a 277x150 pixel advertisement on the right side of the page.

Amazon

My main beef with Amazon is their "Your Account" dropdown menu. Why do I - a visitor to the Amazon home page - need to know that "Manage Your Content and Devices" used to be called "Manage your Kindle"?

Elsewhere, the "Shop by Department" dropdown has seventeen items in it - each with a flyout containing images and as many as nineteen sub-items.

Uline

Uline's homepage is straightforward and visually consistent. But they insist on separating their dropdown menus by "Products" (containing 34 subcategories) and "Uline Products," (26 subcategories) which as a customer seems totally arbitrary.

Also, free offers? Really?

Grainger

My rule: If it's okay that your flyout/dropdown menu obscures a large block of content, then it probably isn't relevant.

Grainger's site is uniquely complicated by the space devoted to its retail stores, but that's no excuse. Just take the top row. They've separated out "Sign in" and "Register now" (two mutually exclusive use cases), and have their marketing email signup (which I don't see why I'd want) right there too.