To upsize or not to upsize images...

There are many people who post here that say one should upsize/resample any image that has less than 300 PPI up to that level because if you do not the printer will do it for you and at a much cruder algorhythm. I have a Canon 9500 printer and my ultimate goal is getting the best 13 x 19 prints I can out of 3 MP images. My Canon rep tells me that the above is not true and that the printer will print what it is sent, even as low as 72 PPI. His argument is that if a print looks good at 170 PPI then it could be hurt upsampling it to 300 PPI. Comparing the prints does me no good as the sharpening varies from a 170 to 300 PPI print and simply confounds the analysis. What is your opinion?