It's often a good idea to leave units in your problem description until you're sure they match up on opposite sides of each equation. This can help you avoid mismatches, e.g.,

Q:  A cylinder has a radius of 6 inches and a height of 50 centimeters.
    What is the volume of the cylinder?
A:  The formula for the volume of a cylinder is 

       V = pi * r^2 * h
    If we just plug in our numbers, keeping the original units, we get
       V = pi * (6 in)^2 * (50 cm)
         = pi * 36 in^2 * 50 cm
         = pi * 1800 in^2-cm
    That _is_ a volume, but it's customary to use a single unit
    of length when expressing areas and volumes.  So in this case,
    we'd want to convert inches to centimeters, or vice versa:
       V = pi * (6 in * 2.54cm/in)^2 * (50 cm)

         = pi * (6 * 2.54 cm)^2 * (50 cm)
         = ... cm^3

More importantly, it can help you catch errors in translating problem descriptions to equations. For example, given a question like

I have 15 coins in a bank, consisting of nickels and dimes.  If the 
total money in the bank is $1.15, how many kinds of each type of coin
do I have? 
it's common for students to jump in with an equation like
d + n = 1.15
without really knowing whether it's correct. Being clear about units can make a big difference:
d dimes + n nickels = 1.15 dollars            Oops!
Here, once the units are in place, it's clear that the two sides of the equation are dealing with different kinds of quantities:
    (d dimes * 0.10 dollars/dime) 
  + (n nickels * 0.05 dollars/nickel) = 1.15 dollars     Better