On sitting on an army empty, that is a tough call and requires some experience and familiarity with the game in question.
In the main, I would not leave it empty if had Riders to fill it. Maybe if MT was quite close or I had nothing but elite Riders, I would wait for Calvs.
But Riders get the same movement as Calvs, so you can still do very well, depending on the game level. I forget what this is, but if it is Monarch (I seem to think that is what this was), you probably are on par with the AI in Tech.
In any event you want to be razing cities or even capturing them at this level you probably do not have to fear their culture any more. The thing is you want to deny them cities and land and lux and of course resources.
What hurts in an AW game (IMO) is not doing serious damage to the AI's. You have to keep shrinking their base and growing yours. This is especially true after Monarch. The bonus offsets your better empire management.
With that in mind, another Rider army could be a big factor in the early Middle Ages, and even more so if it had that 4th unit. It will have 4 moves and cover a lot of ground. IOW you want to be aggressive, not passive in an AW.
I admit, I have only played one AW on a huge map and that was a Sid game on islands, so it does not count. But I would image the game will take a lot of work and you wil be hard press to get the job done before bombers and tanks.
If it does get that far you will be in for some serious new issues. You may not have experienced an AI using a flock of bombers, it is not the least bit of fun. Very painful and very annoying.
Good Luck