I always reapply when reseating a heatsink. Sometimes its not necessary since it was only on there a few hours or less, but I do it anyways. Arctic Silver remains pastey and pliable for a while too so its definitely not 'always' necessary, but I'm always in the habit of reapplying it in case I got any dirt or hair on it in the process of taking them apart (which can accumulate around the sides of the cpu/heatsink as the paste often sqeezes off the edge when the heatsink is initially mounted.) I use alcohol and cotton cosmetic removal pads (that don't come apart too easy) to wipe down the surfaces, and maybe some qtips in alcohol for edges if necessary, and canned air to dry and dust the cleaned surface if necessary.
And when applying the paste, I always squeeze a bit out and work it around with an old credit card into a very thin smooth layer over the cpu. Very little is needed, and I always make sure the entire surface is coated, albeit a very thin layer of it. Motherboard manufacturers usually put a dollop? in the middle and then mount the heatsink, but when doing it manually, its impossible to seat it perfectly straight up and down, which is why I always put a very thin coat over the entire cpu. Getting too much can probably negatively affect heat dispersion, if the coat is too thick and if too much has squeezed out along the sides, since there is heat buildup there too that will be harder to disperse when in a glob of thermal paste.