Calculating marginal tax rates when near a cliff
The marginal tax rate is supposed to reflect incentives for the individual. In order to derive a meaningful metric, the calculation should hence try to reflect the actual decision an individual is facing. An increase in \$1 pay is probably not something people are considering. If, however, someone earning \$10 per hour considers to increase his/her labor supply from 35 to 37 hours per week, you will end up with a pay increase of sth like \$1,000 per year. So I'm rather sympathetic to the \$1,000 step than to the $1 step.  

The OECD also applies a change in hours worked to obtain the change in gross income used for calculating marginal tax rates.