The amount your rip does varies. It's not a for sure amount. Therefore, You would be gambling for either a worse or better dot. The only time it'll refresh rip is when it's about to end.
So, if I have a rip up and its not going to expire soon, then 2 trinkets proc. Your saying it's best not to apply another rip? With the trinkets procced + Dream of Cenarious :S I use addons to track my attack power and it shows I can apply a stronger rip, but the routine doesn't do this.