jamesv,
I'd say your assessment has merit; what I know of current design philosophy would agree with you. However, wing sweep isn't the prime motivator here, as the mach cone is being generated from the point of the nose. What you should be considering, then, is the smallest cone angle (vertex at the nose) that encompasses all the outward extremities of the plane.
Since IRStuff brought up the F-22, I thought I'd look at that (not that I have any great knowledge of the design). I found a fuzzy 3-view on the net and tossed it into CAD to measure angles. A line from the nose to the forward end of the wingtip (in plan view) is the minimum cone angle and comes out at about 29 degrees by my measure. That's mach 2.06--not quite IRStuff's value of top speed, but certainly including a posted supercruise of mach 1.85.
Note also, for interest's sake, that all of the plane in side view was also well within the 29 degree cone.
We're now getting into areas where my memory for this stuff gets all fuzzy, so bear with me... The cone angle doesn't remain a constant as it trails off into the farfield, it flares out to become a weak mach wave (cone angle 90 degrees). There's a cool mpeg floating around on the net of a low-level, high-speed fly-past of a naval fighter (F-14 I think) that really shows this effect off. Anyway, perhaps this flaring out of the mach cone allows the wingtips to remain inside the mach cone up to the quoted mach 2.2 (cone angle 27 degrees). Or, maybe the wingtips stick out in the supersonic breeze past mach 2.06 and the increased drag limits top speed to 2.2. I don't know, but I suspect the former.
Now, where wing sweep enters the picture is after the flow has already passed through the initial shockwave off the nose. As you probably know, with wave angles less than ninety, flow on the back side of the shock will likely remain supersonic, depending on the freestream mach number and wave angle. I don't have any mach tables in front of me, but I think it's a good bet that the post-shock flow in our example is still supersonic. Fortunately though, the flow over the wing isn't dictated by the freestream velocity, but by the velocity component perpendicular to the wing sweep (but I can't remember if it's quarter-chord sweep or leading-edge sweep). So, if the correct wing sweep is selected, the wing can experience sub-sonic flow conditions even in a supersonic freestream. Since the minimum wing sweep for this is based on the freestream mach number around the wing (behind the nose shock), the wing sweep angle can be checked to verify your first design cruise estimate based on the minimum cone angle above. This assumes, of course, that wing sweep isn't set by control, stability, CofG, or structural concerns; which is a pretty weak assumption. Whatever the case, checking wing sweep could give a one-way limit on design cruise speed.
This is all very rusty for me, perhaps someone could further explain/correct the above if I've erred.
The other option for estimating design cruise speed is to search the net and find: "Design Cruise= Mach 1.85". Whichever you prefer
Cheers