There's not going to be any set answer. It will depend on the bowler and the spare left.
Depending on the make up of the league, the majority of bowlers will be in one of two groups.
One group of bowlers will use their strike ball for everything. The other group will use their strike ball for the middle and balance arm side spares, with a combination of strike and spare ball for the ball side spares.
In the minority will be the bowlers who use their spare ball for most all spares
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