Yes — there's no harm at all in closing your outdoor grill's lid while the Flat Top preheats. In fact, it simply helps the Flat Top come up to temperature quicker.

A couple of tips while preheating with the lid closed:

• The lid thermometer reads the air temperature under the hood, not the steel cooking surface itself, so the two can differ. An infrared thermometer is the most accurate way to read the surface temperature, but it isn't required.
• As with all preheating, bring the Flat Top up to temperature gradually and avoid excessive high heat, which can damage the seasoning.