One question I'm often asked by sellers and fellow investors is, "should I rehab the bathroom". And while I'm not a lawyer, I'll give you the standard lawyer response: it depends.
As with most things related to an investment, you have to start with the goal in mind. Here are some questions I ask myself when making this type of decision.
Is this a rental or a flip?
If its a flip, what do other bathrooms in the neighborhood look like? Look at the MLS and see what houses are for sale within a 1 mile radius. Are those homes refreshing bathrooms or remodeling them?
Am I changing the configuration in any way? (Moving a toilet, installing a double-sink vanity, etc...)
Do I have to tear it all out of can I use any of the existing fixtures?
One mistake I've made in the past is just calculating the cost of materials when considering a replacement (bathtub, vanity, etc..). It isn't just materials, its the labor to install the materials and its an ancillary repair you need to do. I once swapped out a vanity without realizing that the new vanity was slightly smaller than the one I took out, so the wall had to be patched around the entire thing. Don't forget labor costs when calculating your estimates!