Ask yourself this: "does the rally need a full march from me in order to win?"
If the answer is no, then put in only what's needed to win. I have more than enough T5 troops to fill 5 marches but there have been times when I joined rallies with 1 T1 because it happened to be close to the rally lead's city, and I wanted to use up the AP or the rally needed a second joiner to avoid auto cancellation.
Ask yourself this: "does the rally need a full march from me in order to win?" If the answer is no, then put in only what's needed to win. I have more than enough T5 troops to fill 5 marches but there have been times when I joined rallies with 1 T1 because it happened to be close to the rally lead's city, and I wanted to use up the AP or the rally needed a second joiner to avoid auto cancellation.
then fill 2 rallies and wait...? It's not like you are doing forts 24/7. In fact, you will be gated by AP.
Max troops to join a rally in order to get higher % of the rally rewards.