If you're shooting groups at the same time as developing loads,I would clean after each group,but don't shoot groups on a perfectly clean barrel either,always have some spare rounds to use as 'foulers'.1-2 should do the trick.
A slightly fouled barrel seems to shoot better groups than a squeaky clean one.
If you're only shooting loads to determine pressure,I would clean after 20 shots or so.
I find it easier to shoot groups,fire over chrono and check pressure all at the same time,you also use less components this way,and you can immediately see which combo is the most accurate and what velocity you're getting.
If you want detailed procedures I use, PM me and I will be happy to give you the best advice on what you need.