Question: When laying hardie backer I was told to make my joints as tight as possible, so I imagine I'll run into this often.

Currently I have a standard 6' board that just barely doesn't fit inside the lip of the tub (I'm leaving a 1/4" gap below per hardie board's instructions).

What is the best way to shave off a piece of hardy board so I can make a tight fit?
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Answer: 
I am not saying this is the cleanest or the most practical but I will just tell you what I do... Angle grinder with diamond blade. I cut these pieces only outside and I wear proper goggles and mask. It is one of the only times that I overdo safety. 

This is the only way I can get a clean small cut in HB. If I use a knife it will take a few scores, then I bend it and since the bent area is small, the break won't be even at all.
You can use one of those cheese grater looking rasps most people use for drywall. They're called serrated contour planes.

Something like the [Stanley Surform Pocket Plane](http://www.stanleytools.com/default.asp?CATEGORY=SURFORM%20TOOLS%20AND%20BLADES&TYPE=PRODUCT&PARTNUMBER=21-399&SDesc=6"%20Surform®%20Pocket%20Plane) or [Stanley Surform Plane-Type](http://www.stanleytools.com/default.asp?CATEGORY=SURFORM+TOOLS+AND+BLADES&TYPE=PRODUCT&PARTNUMBER=21-296&SDesc=Surform%26%23174%3B+Plane+Type+%96+Regular+Cut+Blade)

Those tools will be quicker/easier and make less of a mess than using a power saw. Or at least a more controlled mess. You can also use a utility knife with a sharp blade but the rasps are easier to use and safer.