Depends on the format. If you have to do a presentation platter "culinary salon" stylee and then, say, 10 plated presentations for tasting then you do try to stabilize the coulis on the platter but not on the tasting plates. So yeah, gelatin for salon style presentation only, no gelatin for anything you're gonna eat unless it's bavarian or something. You get it right? :)
One common way to prevent the coulis from running all over is to pipe a dam and put the coulis inside. IMO given your description you should try and make the whole presentation strong, no fuzzy lines with the coulis. Judges like strong lines.