Let's do some math ..  Rounding examples of 7 random hypothetical purchases:
  Case Tag  
Price  
Sales  
Tax %  
Raw Total  
Incl. Sales Tax  
Rounded  
to Penny  
Rounded  
to Nickel  
Rounded  
to Dime  
  Purchase #1 $110.89   10.727%   $122.7852   $122.79   $122.80   $122.80  
  Purchase #2 $211.72   11.293%   $235.6295   $235.63   $235.65   $235.60  
  Purchase #3 $249.39   9.565%   $273.2442   $273.24   $273.25   $273.20  
  Purchase #4 $232.80   13.775%   $264.8682   $264.87   $264.85   $264.90  
  Purchase #5 $241.26   3.358%   $249.3615   $249.36   $249.35   $249.40  
  Purchase #6 $228.51   13.788%   $260.017   $260.02   $260.00   $260.00  
  Purchase #7 $223.88   11.342%   $249.2725   $249.27   $249.25   $249.30  
  7 purchase totals = $1,498.45       $1,655.1781   $1,655.18   $1,655.15   $1,655.20