Regardless of whether a machine appears fine, you should replace the foil between the CPUs and the heat sink the first time you take the heat sink off.

Heat sink compound is always important. The foil doesn't work in the long term because the spring clips are constantly putting tension on the CPU board and over time it eventually warps a little. It doesn't take much to make a gap between the CPU and the heat sink that the foil can't handle, but a pasty material will allow a little gap and still conduct the heat.

Remember, everyone, ALWAYS repaste your CPUs if they have original paste or foil on them, and recheck them every few years.