unless you are having a freeze plug leak problem, I would leave them alone...with full flush and antifreeze change every 2 years and using only distilled water, the freeze plugs will last as long as engine rotating components...
if you rebuild engine, change freeze plugs then, coat inside and out with this or similar product prior to install and let dry...will outlast driver

http://www.permatex.com/products/automotive/automotive_gasketing/gasket_sealants/auto_Permatex_High_Tack_Gasket_Sealant.htm

i use this every fall...many similar anti corrosion water pump seal lubricant products
http://www.acdelco.com/html/pi_vehcare_lub.htm