My question is - will they only request the $490 "Right of Permanent Residence Fee" IF my application is approved...?
OR
Will they hold my application and NOT process it UNTIL they request and receive this fee?
You can wait until they request it. The RPRF generally has been one of the last things requested before an application is finalized. If you receive a request for an RPRF, that does not mean 100% that your application will be approved, but it was a good sign. (I say "was" because it does appear that the sequence of some processes is changing).
They will process your application up until it is ready to be finalized without the RPRF, but it will not be finalized until they receive the RPRF. (in my case, the RPRF was requested along with my passport). But once CIC requests it, you should submit it ASAP.
Paying the RPRF early (before it is requested) is a good idea for applicants who will leave Canada during processing, if they think it will be more difficult to make the payment when requested.