I have built dozens of systems with CDROMs and CD-RW drives on the same IDE channel, and never had a problem in either configuration - Slave or Master. Generally, I assign the CD-RW as the master, but it technically doesn't matter.
I've seen others say they've noticed a difference having the drive as a master, but it really shouldn't matter. Think of Master and Slave as New York and Los Angeles. Neither one is better than the other. They're just two different locations.
There may be some rare cases where a CD-RW drive "likes" to be in the master location, but the cases are RARE and it is wrong to imply that you have to do that...
~cdogg
"The secret to creativity is knowing how to hide your sources."
- A. Einstein