We have discussed this a lot.
https://www.axisandallies.org/forums/index.php?topic=40668.0
is one example. It is not such a good idea to stack it all before Japan moves, but perhaps after they do…depending on what they did and when they declared war, it may be a huge problem for them.
Since Japan usually lands many many air right next to your stack on J1, regardless of what else he does, this can be a really bad move since he gets to smash all your good stuff and you piled it all up for him. All depends on how many ground cannon fodder troops he can bring, which means that a J1 will be quite short of troops and as soon as UK breaks the DoW rule there is no USA entry+money for 3 turns.