Fuel flow is a function of TAS, so for any given holding speed, your TAS will be lower the lower you are.....if you check the numbers on your aircraft, I am sure you will find the fuel flows lower at lower altitude(@ hold speed).
Actually, fuel flow in a jet is more closely related to IAS. While not exactly constant with altitude, it is very close.
Depending on the airplane, higher altitudes (above 15 or 20,000') may require a higher holding IAS due to required stall margins (e.g., 15,000' for the 744). If so, it would be best to be below that altitude.
At max TOGW (~400 T), 744 holding is a constant 286 KIAS to 15,000', and fuel flow varies by 1% in that range. At 30,000', holding is 312 KIAS with approx 10% higher fuel flow.
At light weight (200 T), holding speed is constant 208 KIAS to 15,000', and increases to 222 KIAS at 45,000'. Lowest fuel flow is at 30-35,000'; and is 13% higher at 45,000', 8% lower at 1500'.