Generally, it's advisable to sign up for a trademark surveillance service right after you apply for your trademarks. The validity of your trademark starts from the filing date, not the registration date, and while the examiner evaluates your mark, you already have certain rights. If someone files a trademark application after you, you can raise an opposition against that mark, even if yours is not yet fully registered.
However, trademark surveillance is an ongoing investment, and you might find yourself paying for months only to have your registration refused by the IPO. Some applicants, therefore, choose to wait until their trademark is fully registered to subscribe to a service. It certainly is an option if you accept that you aren't fully protected during the time between filing and registration.
If you are on the fence, what can help with the decision is a professional assessment of your chances of successful registration. If the risk of rejection is low, you can proceed with trademark surveillance right away.