Is it bad to have too little or too much resistance?
There's two ways to do this; serial and parallel.

Serial, you just add the two resistors.  Normally, one would go to a standard reference like this listing to find various 1% resistors available.  Note there is a value (26.7K) which is pretty close to what you need. However you said you are getting parts from RadioShack, so you are constrained to the values they have.

RadioShack has a 500 piece assortment of resistors with quite a few different values.  They are listed here.

Since they have both 12K and 15K values, you can just put them in series and get 27K.

The other way is to put them in parallel.  The formula is:

$$R = \frac{R1 \times R2}{R1 + R2}$$

This is a pain to calculate manually using pairs of available resistors, but a parallel resistance calculator makes it a little easier.  You need to plug in values from the RadioShack table until you get the desired resistance.  I found that 150K in parallel with 33K gives a value of 27.049K.