Theoretically you do not need them but stock springs have no or little sharp edges to machine themselves into the heads.
289hp heads have cast in spring cups.
In my experience, GM's are more susceptible to this than Fords are simply because GM use's a softer grade of cast iron.
Ford uses a high "nodular" content depending on the component being cast.
If you are going to use spring seat cups on a head that never had them to begin with, you usually have to machine the spring seat location with a "spring seat cutter" to get enough room to use them with really any cam with a lift over .500", which by today's standards is 60-70's artifact?
I haven't done an iron set of Cleveland heads in a while (20 years?) and I don't remember there being an issue with the spring seat area being too thin to cut deeper but just when you thought it was safe to go back to the water...?
Just sayin'
If the assembled spring height is inconsistent, assuming all the same parts, look at the valve seats. Some may be cut deeper than others.
Once you go too deep with them, then you need to install valve seats to bring them back up.
The aluminum heads have an advantage here since they already were machined for replaceable seats.