In Carey, Sundberg Advanced Organic Chemistry it is stated:

This preference arises from avoidance of gauche interaction of the aldehyde group and the enolate beta-substituent. The syn stereoselectivity indicates that reaction occurs through an open TS.

The relevant conformations adopted by the two molecules in respect to each other, just before the reaction takes place are attached. The lower one, which leads to the anti aldol, has the energy increasing gauche interaction (the R groups may differ):

Therefore is doesn't matter whether the enolate configuration is Z or E, the syn aldol will be obtained stereoselectively.

In Carey, Sundberg Advanced Organic Chemistry it is stated:

This preference arises from avoidance of gauche interaction of the aldehyde group and the enolate beta-substituent. The syn stereoselectivity indicates that reaction occurs through an open TS.

The relevant conformations adopted by the two molecules in respect to each other, just before the reaction takes place are attached. The lower one, which leads to the anti aldol, has the energy increasing gauche interaction (the R groups may differ):

Therefore is doesn't matter whether the enolate configuration is Z or E, the syn aldol will be obtained stereoselectively.