Should I consider lightfastness scale while choosing artist grade watercolor?
Watercolor is especially susceptible to fading under light. This can be explained by the binder for watercolor and the way watercolor is used. The binder is usually a mixture that contains mostly gum arabic. Due to the usage of watercolor in thin layers the binder will not be able to coat the pigment properly leaving it exposed to light. Lightfastness should therefore definitly be considered when using watercolors. However, it depends on your aim for your drawing. 

If you only want to practice to get better at watercolor you might want to look at inexpensive watercolor pigments that might therefore lack in lightfastness. Those works can still be durable if kept in a dark environment such as a folder. Displaying paintings with colors of low lightfastness will result in quick fading (a hint at how quick the fading will appear is given in this table). If you still want to hang a drawing in a well lit place you can use Aquarell-fixatives to improve lightfastness. Those Aquarell-fixatives or finishes come as a spray and allow to apply a UV protective layer that also protects from dust, moisture and other bad influences. Going one step further you can also frame your image behind UV-protective glass to ensure longevity of colors.

On the other hand, if you plan to display or sell your drawing (from the beginning) you should opt for the (more expensive) lighfast pigments and take the above measure in addition.