I started casting PR colored blanks a few months ago and can say that the short answer is you may or may not need pressure/vacuum if that's all you're going to do(in other words, you're not going to be "embedding" anything in the PR, like snake skins). I found that no matter how careful I am at mixing and pouring, I will occasionally have very tiny bubbles in the finished product(even after gentle agitation), and invariably, one or more will end up on the surface of the finished pen (it shows up as a small "crater" on the surface). For me, pressure was the solution.
I got the HF pressure pot and added a few fittings, caps, and valves (so it will retain the pressure). Now when I cast, I put the mold in the pot just before it reaches the gel stage and pressurize to about 45ppi. I hold it there for about a hour or so before removing from the pot. What the pressure does is squeeze the bubbles in the resin to a microscopic size. When the resin hardens and you release the pressure, they cannot expand back to their origin size thus, for all practical purposes, eliminating them.
You could also put it under a vacuum to degas the resin (i.e. draw the bubbles out), but so far I have not found that to be necessary. Pressure does the trick.
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