My convolvulus is quite large and draping over the wall of a raised bed. I tidy it up after flowering but if there are any straggling stems I prune them quite short whenever I feel like it. I don't know if this is the correct way to treat it but mine hasn't come to any harm.
As for the cerastotigma, my book says to cut out any old or damaged stems at base in March. I would do it now as we have had a late spring. Hope this helps.

I cut my Cerastotigma with the shears every now and again just after flowering or as said by Seabird do it now because of the hard winter. Mine is very large for the type evidently. Perhaps my "pruning" method accounts for it. I don't do it every year.