3S-FE Water Pump: When replace housing?

[jw113]

[Sorry, I should have posted this to the Gen 1+2 forum...]

I have a 1990 Camry SW. My 3S-FE water pump is leaking enough that I'm finding it hard to ignore (though at 440k km I can't really complain), and it's also due for a new timing belt. In the past I've paid the dealer to replace the timing belt but thanks to a couple of great posts here I'm thinking I'll try job myself.

One question I can't find answered, though: It seems you can replace the water pump with or without the housing. What are the signs that you need to replace the water pump housing, and not just the pump alone?

Thanks, in advance!