I have seen it listed as a tactic for Overlords that they can remove certain cards from their basic deck which they feel will be of little use in a given quest or simply to maximise the chances of the cards they want coming out when they draw. I vaguely recall reading this somewhere as being allowed.

However, on re-examining the rules it clearly states that the OL should shuffle the 15 basic OL cards to form his deck. So, am I imagining reading this or is it a viable strategy?

You start with the 15 cards of the basic deck of your choice. As you are able to obtain experience to spend on new cards, you are able remove cards down to a minimum of 15 cards. So if you have three class cards (cards not apart of of the starting basic deck) and you want to remove three basic cards to bring the deck down to 15 cards, the game lets you do that. The Overlord can not do this until he has class cards to replace the original 15.

I hope this helps clear up some confusion.

That makes sense, thanks!