making it up as you go along is fine so long as you are experienced enough to actually make the sessions productive. No one except you can make that call.
One little tip is to keep one exercise in the rotation for every session per muscle group, and use your strength on that to gauge whether or not you're improving.
So say when you train your chest, you decide to always do incline bench. The rest of the routine you do whatever, but you always incline bench. So long as your incline bench is progressing, you know that your routine is doing what it's supposed to do.
That make sense? By keeping in the core lift you know whether or not you're progressing.
Personally, I don't see anything wrong with making shit up as you go in the gym. All depends on the individual.