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Should I always write units in equation no matter if it looks like variable?

I am not sure if it is suitable in the site. I am asking the question here cause Olin and Celtschk said it here.

They told me to use unit in every "single line" (Nope! They didn't say like it. Olin said to always separate number and units. And Celtschk said to write units in middle lines).

I was doing a simple problem, and I noticed units may disturb my equation.

$$S=ut+\frac{1}{2}at^2$$

I am just putting a random value in $a$ and $t$

$$S=5u+\frac{1}{2} \cdot 2 \ ms^{-2} \cdot 5 \ s$$

I can write that 

$$S=5u+\frac{1}{2}\cdot 10 \ ms^{-1}$$

In these equations, ms^{-1} is looking like a value (like as $u$) rather than unit. But if I had put a number instead of $u$ than the equation might look better. But when there's variable the units isn't looking better. What to do in these cases?