Most modern rocket analysis methods can base calculations on either Gibbs free energy or the Helmholtz free energy when computing what the exhaust products (and hence, the exhaust velocity) will be (sources: Sutton & Biblarz, 2010; Gordon & McBride, 1994; Ponomarenko, 2009; one counterexample (but much older): Reynolds, 1986). One requires the assumption of constant given pressure; the other, a constant given volume. When a (well running!) rocket engine is at full thrust, both are true in the combustion chamber, no?
So why is one picked over the other? Wouldn't it be easier to say "I'll have this size combustion chamber" and then figure out the pressure (and thus: throat nozzle) from there? Yet it (from my reading) that assuming a chamber pressure to start is the norm.
Which would be easier to start with--a given pressure or a given volume--if you are trying to develop an engine that will have a certain amount of thrust?