Loen and Canadp has explained well.
Once you get a PR visa, come to Canada,complete the landing formalities, get the PR Card, SIN, health Card etc. If you apply for a US visa shortly after landing, without securing a job or before establishing your ties here in Canada, there are possibilities for rejection of the US visa application, stating the reason that:
"You have not been able to demonstrate sufficiently your strong family, social or economic ties to a country outside the U.S. that would compel you to leave the United States after a temporary stay". .