Whether a probation officer files a violation of probation warrant is generally up to the discretion of the officer. What I mean is that many times a probation officer will not file a violation warrant if a probationer commits minor violations of the terms and conditions of their probation, if the probationer has been doing well with regard to other probation conditions and if the probationer has a good relationship with the PO. However, this can vary based on the individual probation officer. Generally it is best to be honest with the probation officer rather than to have the PO be surprised by a negative result. However, this determination is up to you.
Dirty screen can be sufficient grounds for a revocation of probation. This is up to the Judge that put you on probation. The PO is not required to file a petition for all violations, but must follow the orders of the court of which you received probation. Certain judges take a hard line on drug screens, new arrests and non payments of costs and others do not. The dirty screen won't automatically be a violation, you have a right to a hearing and attorney.
You need to know your PO specific policy on screens, some are more lenient than others. Unless your screens are ordered to be random or on a specific date, it can be within the discretion of your PO to reschedule the date of the screen.