If we're talking fairly small, say 25x16 trunking, then I can't imagine there would ever be much need for clipping even on longer vertical runs, although segregation may be another matter. The cable isn't likely to run straight down, and unless it's extremely heavy it'll naturally want to 'bunch up' (for lack of a better word) and stay in place simply by friction against the sides, back and lid of the trunking.
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