Reset SMC with absent removable battery
I often use my Mac without its battery to avoid overcharging during
  long periods of plugged-in use.


(Why is this even a thing?)  The SMC that you are attempting to reset manages the flow of current to/from the battery to ensure that it's not overcharged.  There's dedicated charging circuitry specifically for the task of safely charging your battery.

Additionally, all modern battery packs have over/under charge protection circuitry built into them.  So, removing your battery "during extended period of plugged-in use" is completely unnecessary and you risk damaging things if your battery is the non-removable type (those connectors weren't designed for repeated connects/disconnects).

To answer your question...


  Could I still reset the SMC by skipping the battery removal step and
  just unplugging the MagSafe adapter and holding the power button for 5
  ~ 10 seconds?



If your battery is removable - this would be a moot point given that you're supposed to remove it.  Reset the SMC and plug the battery back in.
If your battery is non-removable, plug the battery back in and reset the SMC. Leave the battery plugged in.


There are reasons to reset the SMC and this shouldn't be done in the course of "normal maintenance."  Too many times folks react to common errors with "I need to reset the SMC/NVRAM) - this is hardly the case.  Please see Apple's support page:  Reset the System Management Controller (SMC) on your Mac for more information on Indicators that the SMC might need to be reset.