sanding old plaster wall texture to prepare for mudding
I assume you have a textured surface.  If so, yes sanding the peaks down a bit will help later on with the skim coat, because less skim coat will be required to fill in the uneven spots.

But there are things to consider.


Plaster dust gets everywhere, so sanding creates a big mess
Plaster is very hard, so it is hard or harder to sand that straight mud or joint compound
Plaster may contain asbestos if it is old, so you might want to test first before sanding


I would start with 120 grit paper and then move to 220 grit for finishing  Since plaster is hard, if you are not getting anywhere with the 120 , move to a 60 or 80 grit if necessary.

For sections that stick out the most, you may be able to scrape them off as opposed to sanding.

A power sander will be quicker than manunaly sanding with a standard block or pole  drywall sander.  I know some of the power sanders for drywall have vacuum attachments which may be wise to rent one to keep the dust down. 

Also, with this type of job definately wear mask and tape off other areas of the house prior to sanding.  I usually try to leave windows open for fresh air because it gets really dusty.