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 $139.98   4.594%   $146.4107   $146.41   $146.40   $146.40  
  Purchase #2 $110.52   13.728%   $125.6922   $125.69   $125.70   $125.70  
  Purchase #3 $75.58   8.689%   $82.1471   $82.15   $82.15   $82.10  
  Purchase #4 $144.25   6.454%   $153.5599   $153.56   $153.55   $153.60  
  Purchase #5 $18.87   4.757%   $19.7676   $19.77   $19.75   $19.80  
  Purchase #6 $84.85   12.861%   $95.7626   $95.76   $95.75   $95.80  
  Purchase #7 $17.88   11.354%   $19.9101   $19.91   $19.90   $19.90  
  7 purchase totals = $591.93       $643.2502   $643.25   $643.20   $643.30