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 $57.98   9.315%   $63.3808   $63.38   $63.40   $63.40  
  Purchase #2 $234.99   12.867%   $265.2262   $265.23   $265.25   $265.20  
  Purchase #3 $115.52   13.115%   $130.6704   $130.67   $130.65   $130.70  
  Purchase #4 $216.63   6.287%   $230.2495   $230.25   $230.25   $230.20  
  Purchase #5 $31.66   12.243%   $35.5361   $35.54   $35.55   $35.50  
  Purchase #6 $87.63   6.359%   $93.2024   $93.20   $93.20   $93.20  
  Purchase #7 $163.39   13.237%   $185.0179   $185.02   $185.00   $185.00  
  7 purchase totals = $907.80       $1,003.2833   $1,003.29   $1,003.30   $1,003.20