It's a slight detour, but when you consider that the fuel prices in Luxembourg are noticeably lower than those in surrounding countries (government controlled in Lux), it's not a bad place to pass through. If you are going to Bastogne, it's roughly an extra 70 miles onto your trip if you go through Luxembourg. Price comparisons for 98 unleaded (per litre) are:
Luxembourg: 1.28EUR (or £0.93), Belgium: 1.57EUR (or £1.14), Germany: 1.49EUR (or £1.08) France: 1.48EUR (or £1.07).
Entirely up to you of course, but I would say it's worth the extra 70 miles!