Whether or not the ears will stand is down to a mix of factors. Amongst them is getting a good vet who knows what he's doing (i.e. is experienced - ask some breeders for recommendations). Then you should get decent advice that gives you the best chance of success. Other factors are the ear type of the puppy (some have soft cartillage that doesn't lend itself well to crops, others have stiffer ear cartillage that is relatively easy to get to stand), the type/length of crop you get, and the amount of time/dedication you're prepared to put into making them stand. On the last point - bear in mind that it can take months of taping, even up to a year in some cases.
Bearing in mind that your "window" of opportunity for having ears cropped is only until the pup is 12 weeks of age, if you really want to do it, it would be wise to find a good vet who can/will do it well before getting the puppy.
In fact, in Europe, Canada, etc. you cannot show a boxer if the ears are cropped.
Not exactly true. In many parts of Europe, cropping surgery is illegal - you just can't do it, and in the
UK (where cropping was made illegal well over 100 years ago) you cannot show any cropped animal. But in most other European countries, you can still show a cropped dog if its home country still allows cropping (and, obviously, there is no problem with cropped dogs at shows in those countries that haven't banned the procedure). The same applies to tail docking, btw
Irrespective, you are quite correct in saying that ear cropping is a rapidly diminishing practice worldwide as more and more countries ban the practice (and even in those where it isn't yet banned).