2D Will Never Die Tutorials - Photoshop Required?

After reading through the 2D Will Never Die tutorials I'm reconsidering whether or not I will need Photoshop when it comes time to actually produce sprites for my game.  Almost all of these tutorials include Photoshop tips and tricks for improving both the quality and output of your sprite work.

For now, as I'm literally learning from absolute zero, Pixen should do just fine.  But when it's time to start cranking out creatures and levels for my game I may need to revisit these tutorials and pick up Photoshop.  Better start saving my $ now...

Key tips that apply regardless of software package from the 2D Will Never Die tutorial series includes:

• Step 1, line work.  Step 2, basic flat coloring.  Step 3, shading.

Hmm, well most everything else seems to be Photoshop specific.  I know there are some great tips in there about using grayscale to evaluate lighting, but I can't seem to find a non-Photoshop specific instance of it.