In the beginning of Sri Chandi DI VAAR WHICH IS also the beginning of Ardas we are advise to contemplate/meditate on the first 9 gurus by doing so we receive cetain boons. For example, through SRi Guru Harkrishan all our pains go away, and through Sri GUru Tegh Bahadur we receive the gift of "nau nidh". Generally people do ardas to ask for both material and spiritual gifts and only Sri GUru Akal Purakh can answer our prayers. We see no difference between Guru Sahiban and Sri AKal Purakh thus when we meditate on each Guru we are in reality meditating on Sri Akal Purakh as there is no difference between the two.
When we say Dhan Sri GUru Nanak Dev JI or Dhan Sri GUru Tegh Bahadur Sahib JI then their is no harm because when we praise GUru Sahiban then we are in reality praising Sri Akal Purakh Sahib, but we are advised not to do simran abhyias on any of the gurus names the way Hindus do simran on raam and har krishan. Guru Sahiban came to spread the one true name and not their own name.
ਸਤਿਗੁਰਬਚਨਕਮਾਵਣੇਸਚਾਏਹੁਵੀਚਾਰੁ॥