It doesn't really matter when you convert to B&W. It's a personal preference based on your intent and conversion methods.
For me, if an image is never going to be color, then I'll convert in the raw converter if the image is in Lightroom. If it is in ACR I'd be more inclined to convert it after its been developed using the channel mixer. If I do any cloning, I'll do so to the color layer. I typically do most tone adjustments in the raw converter and then fine-tune if needed after B&W conversion.
Since your just starting out, you might have fun trying out the many types of conversion
available in Photoshop while you experiment.