IMO, a player should try and have their fleet upgraded by mid-region, this is easier R4 and above, then start saving keys, parts and coins for the next region. The caveat to this is if you slow down and play multiple events during a region you can get stuck with event trains that you are not able to get upgraded before the next level, on this you can do them later to save for the new units in the next region.
At the end of the Japan Event I went from R7 to R8, started the region with only the event trains and was able to get almost all of them upgraded before running a single job( ran out of coins). My current R8 fleet is fully upgraded with 2 1/2 events worth of trains and all of the region units and only 1 duplicate.
Coins, epic and legendary parts are always harder to come by so saving them for the next region is helpful but it takes self control not to use them. I will have to take some of my own advice with a grain of salt because I am past the mid-region but not close enough to get to the next level before the end of the current event, hopefully the next event has plenty of parts to take me to the next region with a good amount to upgrade those event trains. Also I have restrained myself from doing blue tops in R8 as I have only done 1 by accident, did get the story line complete before the Seaport though.