Why should I put a 10K resistor with an LDR?
LDRs themselves come in a considerable range of resistances. Search LDR specs and you will see what I mean. For example, from eBay I found one:


  Bright Resistance(10Lux)(KΩ):20-230
  
  Dark Resistance: 2 MΩ


This particular one, for example, seems to range from around 20k to 2M ohms. A reasonable series resistor would then seem to be somewhere in the middle of that range, otherwise all your readings would be skewed at one end of the scale.

For example, in this case if you used 10k and the LDR had 20 k resistance in very bright light, then the voltage would be 3.33 V

20000 / (20000 + 10000) * 5 = 3.33 V


Then if it got slightly darker and the LDR was 230 k, you would get:

230000 / (230000 + 10000) * 5 = 4.79 V


And if it was really dark you would get:

2000000 / (2000000 + 10000) * 5 = 4.98 V


You can see from those figures that you will never get a reading less than 3.33 V with this particular LDR and a 10k resistor. Thus a higher value resistor would give you a better range of readings.

However this totally depends on what resistance your particular LDR has.