While I do think the grandparent is being a little too conservative, there is no "trying and getting rejected" here. This is not a credit that you apply for, it's part of your income tax filing. The only case where it would be reviewed is if you were audited by the IRS, and if they then disallowed your credit, you would be liable to repay that amount, plus interest and potentially plus late-payment penalties. AND, you would likely be flagged as a target for future audits ...

In general, when you claim a deduction on your US income taxes (personal or business) you want to be pretty darned sure that the deduction is going to stand up to an audit.