Depending on how much you are improving the case, the velocity and accuracy will not change much fire forming using a full charge and shooting vs. already formed case. When I fire form anything I load a near max charge and go shooting. When using the method you ask about, you usually don't get the large cases fire formed all the way. Better off getting bullets down range.If the rifle is new to you it lets you get accustomed to it that much faster. And its a whole lot funner than messing around with cream of wheat.
When finished you can then work up a load and adjust your dope accordingly.