Finally a thread I can actually add something too! I've been in the trade for 25 years, and here's my take. If it was my building, the easiest legal thing to do would be to replace the plugs and switches with CO/ALR devices like you have been doing. Yes, they are more expensive. But they are approved for attachment to aluminum wire. Even the cheaper grades are approved, if they have a UL listing. I'd also guess, that even using cheaper devices will be better than the old ones you have now, as far as safety and longevity.
Aluminum wire has 2 big draw backs. First, it expands and contracts much more than copper or steel, due to temperature. This creates loose connections which leads to heat, and possible fires. Secondly, Aluminum wire breaks easily when twisted or bent sharply. For this reason, using wire nuts is not an approved method in some areas of the country. Even the high pressure crimps are a pain when you have to fold it back into a box .
There are wire nuts approved for use on copper/aluminum connections which come with a dab of oxidation inhibitor, but care should be taken not to twist 'em so much as to break or crack the Aluminum. Keep in mind that the compound used is for corrosion protection, and has no effect on the expansion and contraction of this connection
Aluminum wire is still legal to use as main service feeders. This is because the wire is so large, you can actually put a wrench on the connection to get it sufficiently tight.
If you continue to change out the devices, take note of the insulation next to the screw terminals. If the insulation is not dis-colored or hard and brittle, then you know the connections were good. If you run across any suspect wires like this, cut them back, length permitting, to a point with good insulation, re-strip and attach. Make all your connections as tight as you can, short of breaking the device. Potential "bad" connections, sometimes are detectable by feeling the recptacle and wall around it to see if it warmer than surrounding areas. Bad terminations could be at any plug or receptacle, in-use, or not.
The last bit of advice would be, if this is a residence, I would consider putting any circuits in the bedrooms, on AFCI (arc fault circuit interruptor) breakers. This type of breaker has only been required for bedrooms, copper or aluminum, since 2002. They aren't cheap ($35), but you'll probably only need 3 or 4. They look the same as a GFCI breaker, but serve a totally different purpose. If you have a dangerous amount of arcing going on in a switch, plug, or light fixture in the bedroom, it will trip the circuit until the situation is fixed. The AFCI breaker would be a "must" for me, if it was my house. Worst time and place for a fire is when you're asleep in a bedroom. This is why smoke detectors are also required in every bedroom doorway area.
Sorry I went on so long