Cottered cranks are pretty indestructible and very very reliable. The system is a pain to disassemble but frankly they are less likely to have problems than alloy square taper cranks. I've taken apart and assembled a number of them and never had any problem. I don't have the nice press, I've made do with a brass punch for removal and a wooden mallet for insertion. Ruined a cotter during removal that way once, but it was on a Peugeot and I have a bag of cotter pins so I just put another one in.

In any case, it would be really hard for them to slip. Even installed horribly, as long as you can see that the cotter has the nut screwed on there's just not much to worry about.

Cheers,
Karl