Is registering the path 'node/%/ajax' bad practise?
I wouldn't say it's bad practice as such, but I always go under the assumption it's good practice to prepend paths with your module name, e.g.

node/%/mymodule_ajax


That way you can be pretty sure that you won't run into any issues with other modules that don't follow the same convention.

For the record I don't think I've ever come across a module that implements the node/%/ajax path, but if you're at the start of your implementation and can change the path easily, I'd go with adding your module name to it.

It'll mean some extra work on the JS side, but if you want to be as sure as possible that you won't get path conflicts in the future, I think it's your best bet.