Thanks for the excellent question.
Currently, both PayPal( see UPDATE below ) and Authorize.Net do NOT support tax on recurring payments, at least not in any official way. The documentation for these Payment Gateways, states that: tax, if any, on recurring payments should be included in the total price calculation.
UPDATE. PayPal Pro integration now DOES allow tax to be calculated separately by s2Member, even for Recurring Billing Profiles, and s2Member has been updated to support this as of s2Member Pro v1.5.8+.
So, while s2Member DOES make it possible for you to charge tax on recurring payments, the information stored in your accounting system for each recurring payment is represented by one total amount. In the event that you need to change the tax rate, here are your options, as I see them.
1. First, change your Tax Configuration for s2Member so that all new sales incorporate the proper tax calculation at the time of checkout. This way all future sales are calculated properly. You can access this in your WP Dashboard, under: s2Member -> PayPal Pro Options -> Tax Configuration.
Handling existing Customers.
1. You can log into your Payment Gateway and modify the total price of the next rebill amount. There you can modify the total amount being charged on a recurring basis. Either increase it or decrease it. This is troublesome though, because it has to be done manually, one at a time. It can also be difficult to calculate these properly, because the amounts are not broken down, it's just one total amount. ^ In addition, the Customer might be surprised, because now they are being charged more or less than they originally agreed to pay.
2. Recommended. Simply change the way you report your tax collection. In other words, if existing Customers are being charged $29.54/mo, which includes the tax you were charging at a particular point in time. If that tax rate changes, report the amounts differently in your accounting. In other words, if you were taking $28 of that $29.54 before, now you might have to take a hit. If your tax rate goes up, you might keep only $27.85 instead of the full $28 you were getting before. While this is not ideal, it's the best option available ( in my opinion ), and it avoids any confusion for the Customer. After all, the most important thing is not to lose the Customer over a change in tax rates.
Related thread: https://www.x.com/message/190928#190928Statistics: Posted by Jason Caldwell — April 25th, 2011, 6:48 am
]]>
. 
. 