Are synaptic boutons always located on axons?
Axonal boutons are named as such due to their shape. Dendro-dendritic synapses do not resemble this shape and are never referred to as boutons. It does not appear that synaptophysin is expressed on dendrites (Fletcher et al. 1991), but I am unaware of a study that targeted dendro-dendritic synapses specifically.


References
Fletcher, TL, P Cameron, P De Camilli, and G Banker. ‘The Distribution of Synapsin I and Synaptophysin in Hippocampal Neurons Developing in Culture’. The Journal of Neuroscience 11, no. 6 (1 June 1991): 1617. https://doi.org/10.1523/JNEUROSCI.11-06-01617.1991.