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 $37.77   12.208%   $42.381   $42.38   $42.40   $42.40  
  Purchase #2 $144.46   3.675%   $149.7689   $149.77   $149.75   $149.80  
  Purchase #3 $217.92   3.778%   $226.153   $226.15   $226.15   $226.20  
  Purchase #4 $104.67   3.317%   $108.1419   $108.14   $108.15   $108.10  
  Purchase #5 $43.30   5.221%   $45.5607   $45.56   $45.55   $45.60  
  Purchase #6 $89.91   8.476%   $97.5308   $97.53   $97.55   $97.50  
  Purchase #7 $197.40   6.292%   $209.8204   $209.82   $209.80   $209.80  
  7 purchase totals = $835.43       $879.3567   $879.35   $879.35   $879.40