The spacings I recommend are for long term growth. I have found that closer spacings result in some plants being corwded out as roots of larger neighbours take over.
However if you plant at 18 inches then it does give more early harvest, before some plants fade.
Another aspect is that asparagus roots greedily to its side in summer, for moisture especially, so you are right that a staggered spacing is a good idea, and not too close to your bed edges.