In a closed system, a good place to use expansion tank is at the fill valve. A bladder style tank precharged at 12psi and coupled to fill valve, air eliminator andvent (see Fill-Trol) Bladder style is better as others inevitably introduce air into radiators.
A 10-degree drop is OK. Especially when radiators are already heated up. When thermostats are just beginning to call for heat and radiators are cold, a 60-70 degree difference is more likely.
Now, about water heater expansion tank. Gotta say, meter explanation is new to me. Most places, code mandates a backflow preventer (which usually is combined in one unit with pressure regulator). A backflow preventer is also usually required before boiler fill valve (so that boiler water does not return to potable supply) and is recommended before a water heater.
Now, as water heats, it expands. If hot water is used regularly, lackof an expansion tank is no biggie - the effect is maximized when a lot of hot water is used up, followed by a period of nonuse. In a system where pressure is already fairly high 110+psi), this can lead to dangerous pressure buildup. No BF preventer before heater means that you also have your cold system to buffer that pressure - still not the best of solutions.
for info on tanks, and to size your tank. Installation is fairly simple - you put a T into the supply pipe, adda short nipple, a threaded adapter (female) and then connect the tank.