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 $234.52   6.286%   $249.2619   $249.26   $249.25   $249.30  
  Purchase #2 $76.31   12.350%   $85.7343   $85.73   $85.75   $85.70  
  Purchase #3 $154.47   12.497%   $173.7741   $173.77   $173.75   $173.80  
  Purchase #4 $155.35   6.554%   $165.5316   $165.53   $165.55   $165.50  
  Purchase #5 $26.65   11.621%   $29.747   $29.75   $29.75   $29.70  
  Purchase #6 $214.29   3.665%   $222.1437   $222.14   $222.15   $222.10  
  Purchase #7 $52.26   9.785%   $57.3736   $57.37   $57.35   $57.40  
  7 purchase totals = $913.85       $983.5662   $983.55   $983.55   $983.50