Q:

Do I really need to care if my car burns oil?

My car, a 97 Mazda 323, burns about a liter of oil per 1000 kilometers, mostly start and stop city driving.  Sometimes I see a puff on start-up and sometimes I see smoke while accelerating, but it's intermittent. So I figure that both the valve seals and the oil rings are probably worn ( even though the car supposedly had a head job about four or five years ago).
My question is, to what extent does this really matter, beyond the cost of the oil burned?

A:

You said "valve gaskets" but I'm going to assume you actually mean the valve seals. Since you are seeing a small puff of smoke sometimes during startup and when you are accelerating, these are usually caused by two separate issues, both of which you are mentioning. 
Start-up puff of smoke can usually be attributed to bad valve seals. This is because oil will leak past them after engine shutdown. It then either pools on the back of the valves (if closed) or drips down past the valves onto the piston (if closed). When you start up the car, the oil burns off and causes the puff of smoke. 
Smoke on acceleration can usually be attributed worn oil control rings not doing their job and allowing oil to pass by them and into the combustion chamber. 
One of the issues which you could be causing is that the oil will eventually plug your catalytic converter (if it's still installed).

