If you file your application as an intent-to-use (1b), you will have to prove your commercial activities on the US market within about a year of filing your application.
Once your application has been examined by the US examining attorney and no issues have been found, you will receive a notification. After your application is issued a 'Notice of Allowance', you will be asked to provide a specimen (proof of your sales/commercial activity on the US market). When submitting the specimen, you'll also need to pay an additional fee.
If you postpone the filing of your application until you are active on the US market (use in commerce filing (1a)), you can avoid the aforementioned fee. However, the trademark protection will start later.
Ultimately, it is up to you when you want to submit your application; however, it is generally advisable to file as soon as possible.