I’ve been in the garage experimenting and have found:
Cable cutters: leave a sharp edge, which is what this is all about.
Nail clippers: couldn’t get low enough so they still left a sharp edge, not as bad as cable cutters, but you still cut yourself. Finished off with a needle file and the finish was perfect.
Side cutters: not bad. I couldn’t get them into the end of the tie so there was a still a small sharp edge. Finished off with a needle file.
Twisting: the best initial results but it leaves a raised bobble which could slice a finger. Finished off with a needle file.
Toenail clippers: found these in the bathroom cabinet, the ones that look like small pliers. Got nicely into the end of the cable tie and left a smooth finish. The quickest out of the lot as no finishing with a file needed. I’ve just been round all the bikes and trimmed the cable ties with them.