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 $191.37   12.812%   $215.8883   $215.89   $215.90   $215.90  
  Purchase #2 $106.70   9.554%   $116.8941   $116.89   $116.90   $116.90  
  Purchase #3 $221.38   11.303%   $246.4026   $246.40   $246.40   $246.40  
  Purchase #4 $59.94   7.734%   $64.5758   $64.58   $64.60   $64.60  
  Purchase #5 $25.84   4.817%   $27.0847   $27.08   $27.10   $27.10  
  Purchase #6 $226.30   4.524%   $236.5378   $236.54   $236.55   $236.50  
  Purchase #7 $27.70   12.538%   $31.173   $31.17   $31.15   $31.20  
  7 purchase totals = $859.23       $938.5563   $938.55   $938.60   $938.60