I currently have 3 credit cards, one of which I virtually never use.
I'm planning on calling the companies and asking to change the billing
cycle so they end on the last day of each month. This would line up
with my rent so I can pay everything at once.
In my experience most companies are flexible regarding the end of the billing cycle, and the resulting due dates.
The ones that weren't flexible were utilities that required a person or device to read the usage. This could be gas, electricity, or water. They wanted to read everybody in the neighborhood on the same day. As we move towards smart meters this requirement may go away.
You may find that you don't want the billing cycle to be at the end of the month, you want the due date to be early in the month so that you can send in all the payments at the same time. Most companies have a several week grace period between the end of the cycle and the due date. If the cycle ends at the last day of the month you may find that the bill isn't generated until a few days later.
Some bills you may know exactly what the bill will be. In other cases you won't know. Utilities vary by month. So do other bills such as your credit cards.
Is my reasoning sound? Is there any downside? Now that I think of it,
why do so many bills fall on random days of the month. Why isn't it
common (like with leases) that payment is due on the first or last day
of the month?
If you are paid once a month or twice a month then the timing of the bill during the month can allow you to get into a rhythm. But if you are paid every two weeks that rhythm with monthly bills doesn't last.
Why do companies spread the due dates out? It allows them to get money throughout the month. They also have expenses and getting all income on one day gives them money management problems.
I want to cycle back to an earlier statement:
asking to change the billing cycle so they end on the last day of each month.
While many companies will be flexible, that doesn't mean that all dates are possible. They might not make the end of the cycle or the due date be the last day of the month because that could be the 28th, 29th, 30th, or 31st depending on the month. They also have to factor in weekends and holidays. What do they due if the key date falls on a Sunday, they might not want to send the meter reader out on a weekend.