Is there any good reason not to increase upload_max_filesize and post_max_size?
It is mostly a problem if you have limited ram and multiple users doing uploads at the same time. Both of those are per session, so a user starting a slow upload can eat up quite a bit or resources for you for a long time.
If you have over 40 or so users doing uploads/big posts at the maximum size you will run into problems. Otherwise it shouldn't be any issue increasing those variables.