I usually just combine all my cards I get right away but sometimes its smart to be more tactical. Some of the old cards don't have many on the market so its worth looking at what levels there are to combine. Like sometimes you are 1 BCX short of maxing out and all the cards on the market have already been levelled up to 5+ BCX.
btw You can still over shoot the max but now there is a warning.
All very good points. It's always best to check the market before combining if one doesn't have the correct amount of cards owned already. Even if one does have the correct amount owned its still a good idea to check. There might be some deals on already leveled cards. Thanks for pointing out that there is a warning message. Very useful for those worried about over-combining.