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 $51.81   4.756%   $54.2741   $54.27   $54.25   $54.30  
  Purchase #2 $241.98   12.672%   $272.6437   $272.64   $272.65   $272.60  
  Purchase #3 $122.79   5.819%   $129.9352   $129.94   $129.95   $129.90  
  Purchase #4 $88.87   11.720%   $99.2856   $99.29   $99.30   $99.30  
  Purchase #5 $113.71   7.619%   $122.3736   $122.37   $122.35   $122.40  
  Purchase #6 $150.77   10.484%   $166.5767   $166.58   $166.60   $166.60  
  Purchase #7 $135.68   9.872%   $149.0743   $149.07   $149.05   $149.10  
  7 purchase totals = $905.61       $994.1632   $994.16   $994.15   $994.20