In tournament: Never.
In casuals I do it in certain conditions: Let's say I'm playing against a player, who is couple or one level lower than me. When I'm playing bad and he is playing good, the matches are pretty even, he might even win some of them. Then, when I'm playing good, he get's 4-stocked. If I notice, that everytime when I get my own game on point and start beasting him down so that 4-stock or something might come, he starts to make suicides (either accidentally or like semi-purposely), then I will suicide.
The reason I do it is because then he cannot explain his total destruction with suicides or anything. Like: "don't put that match on youtube, I played bad, I even suicided...". I just want that he cannot explain a single part of me beasting him with he making a suicide. It irritates me that if when I get on point with my game, the opponent starts to make suicides easily, so my own suicide then is like a counter to his :D. And I have seen, that when I counter their suicide with mine, it irritates them, and the result is, that they start making less suicides when I'm playing good, because they know, that I will suicide also, and it doesn't do any good with them.