The original poster and some of the people who replied are talking of two different things.

Freewheels were screwed upon the right side of the hub, and quite frequently the threading was a bit squewed, making for a very wobbly freewheel.

Cassettes, on the other hand, are inserted in a spline and should be almost straight. I said "almost" because it's not micrometric precision. But a few factors may make it crooked:
– sand on the freehub (i.e. the base onto which the cassette is inserted), which would prevent it from sitting squarely;
– not installing the last cog in its proper spline (i.e. small notch vis-à-vis small ridge), which means the lock ring isn't seated properly on all sides.

Both problems ultimately mean that the cassette is not properly seated and may be a wee bit loose. It will then loosen itself, especially if you ride on cobblestones. And then you'll have problems.

So in a nutshell, a slightly wobbling cassette is not a problem, but a loose one is.



P.S. The move from freewheel to cassette was done around 1987-1995, depending on the value of the bike (i.e. more upscale bikes moved sooner)