Question
Should they be topped to make them spread out instead of getting taller?

Answer
John,

Like almost all plants, Arbor vitae has a built-in size it wants to get to, both high & wide.  Different varieties of it have different size limits.

Topping it can cause what's left behind to produce denser growth, but it won't cause the plant to grow wider than it would have normally.  More often than not, that denser new growth will be mostly within a couple feet of the top.

Light available, age of the plant, and what percent of the plant you are taking off all play a part in what happens to what's left behind.

If the A.v. is pretty old and woody, and by topping it, you get significantly into the older woody section of the plant,  it may stay bare for a long time (years).

For what it's worth,
Mark in Portland