Answer: There are a couple of things to consider when you plant along a property line. One is, where exactly is the legal property line? You need to find the surveyors' stakes or markers to be certain because sometimes over the years the line has "shifted." You have to be sure it is planted well on your own side so there is no dispute twenty years from now for example about who owns the hedge and or should be caring for it or who has the right to remove it. In most cases too anything that encroaches over the line can legally be removed (cut off) by the neighbor -- and you may not for instance like the way they prune it, meaning it can become a real issue between neighbors. Best to avoid that possibility.
Next, one of the nice features of hollies is that they tend to have a fairly uniform natural growth habit. If you allow it ample space to grow and mature, you might not have to trim very often, just remove the occasional lopsided or wayward branch or tidy it up after storm damage. A natural hedge is far less work in the long run than a clipped one. Another feature of hollies that is ruined by severe clipping is the berry display. The plants bloom on old wood the in spring and berries follow on the spot of each bloom; if you prune it back severely you will mostly prevent the flowering and/or fruiting.
Keep in mind that the mature sizes listed are an approximation, so depending on the growth conditions where they are planted they may exceed that size, or not quite reach it. Typically for a line of shrubs to meet at the tips without too much crowding you would plant them about as far apart as their expected mature width. For a clipped hedge you would plant them closer together, depending on how wide you could let it get (you want it to be somewhat in proportion to height as well) and how long you can wait for them to grow together. In my experience they seem to be healthier in the long run at a wider spacing if you have the room to plant that way. Finally, pruning it narrow will not force it to grow taller than it would normally.
I hope this helps.
Q&A Library Searching Tips