Yes, definitely change it. It's just below the heater hose outlets in the firewall, on the driver's side. Look down there with a flashlight and you'll see it. Two clamps hold it to the heater hoses, and a vacuum line runs to the top. Easy change with maybe a little tugging to free it from the hoses. It's also a good time to inspect the hoses as well, although they tend to last longer than five years.

my car is a 945,,,,1995

Isn't in the engine compartment, near centre line and high on the firewall in the 700/900 series? Seems to me it was trivial for my mechanic, it certainly was cheap. He did it while doing an oil change, and didn't bother calling me for the go-ahead when he found the leak so I assume it was only a few minutes work.

We would have left it for me except that in his experience, once they start leaking they can crack anytime.

Not sure whether you have a 940 or 960 series, but on the 240's the heater control valve is up underneath the drivers foot well kickpanel, up against the firewall by the heater core. If you have a 940, it is probably there. If you have a 960, let me know where you found it. Ordinarily, I would not fix this unless it is leaking, which you would know about... you wuld have coolant at your feet or in your carpet nearby. I don't think the latter models fail like the early 240's. It isn't to hard to replace, given they are over the footwell, but I know that Volvo engineers, in their infinite wisdoml had them assembled (probably before it was dropped into the car) with the mounting screws facing the firewall. One of those 90 degree screwdrivers comes in handy.