I definitely think you're doing the right thing saving the slogans etc. Its always a matter of personal choice and judgement, but here's some things to ponder...
The stamp club members might enjoy exploring this area of collecting too. Imagine - if they have a cover from the 1970s in their collection, by the time they are my age (quick mental arithmetic here


) the cover will be 70 years old...
Tha stamps on these covers may well be common ones, and you will end up with many duplicates Having some "on piece" makes a common stamp more interesting (to me, anyway!) and can help put them in their historical context.
Perhaps you could let the club members have a look through and choose some covers they would like to keep before you cut any more?
Covers can add another dimension to a topical collection.
Just some random thougths from someone who regrets cutting a lot of covers in the past!