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 $64.67   13.283%   $73.2601   $73.26   $73.25   $73.30  
  Purchase #2 $70.83   8.627%   $76.9405   $76.94   $76.95   $76.90  
  Purchase #3 $133.52   7.352%   $143.3364   $143.34   $143.35   $143.30  
  Purchase #4 $246.80   8.830%   $268.5924   $268.59   $268.60   $268.60  
  Purchase #5 $35.39   7.761%   $38.1366   $38.14   $38.15   $38.10  
  Purchase #6 $179.48   5.749%   $189.7983   $189.80   $189.80   $189.80  
  Purchase #7 $133.77   3.830%   $138.8934   $138.89   $138.90   $138.90  
  7 purchase totals = $864.46       $928.9577   $928.96   $929.00   $928.90