I have been giving this a lot of thought in recent months. I use different chairs at home, in lessons, when playing with others, etc. I reckon the most important is my daily practice sessions. I was using a dining room chair which did not quite keep my thighs vertical, and that was sometimes a bit of an issue. I have now just ordered an adjustable orchestra chair (from Black Cat Music). This allows each leg to be adjusted individually, allows the seat to be sloped to taste, and allows the back to be positioned as needed. I hope this will improve the efficiency of my practice sessions.