when adjusting the valves, do the pistons have to be extactly at tdc or u can be off a little?

When #1 is at tdc, the arrow on the back of the cam sprocket is suppose to point up right? #3 is at tdc if the arrow is pointing toward the front of the car right? I'm just confuse about setting tdc to the piston. Haynes tells me to turn the crank pulley 180 for #3 tdc but how do I know exactly where that is without any mark? The manual assume that my timing belt cover is off so I can see the word "up" on the cam sprocket but I don't. All I can see is the back of the cam sprocket and a single arrow.

btw, can a valve clearance be tighten due to vibration over time or only loosen is possible?