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 $244.77   13.870%   $278.7196   $278.72   $278.70   $278.70  
  Purchase #2 $102.81   5.225%   $108.1818   $108.18   $108.20   $108.20  
  Purchase #3 $98.99   9.734%   $108.6257   $108.63   $108.65   $108.60  
  Purchase #4 $215.37   9.729%   $236.3233   $236.32   $236.30   $236.30  
  Purchase #5 $76.53   11.138%   $85.0539   $85.05   $85.05   $85.10  
  Purchase #6 $97.74   12.481%   $109.9389   $109.94   $109.95   $109.90  
  Purchase #7 $189.65   11.321%   $211.1203   $211.12   $211.10   $211.10  
  7 purchase totals = $1,025.86       $1,137.9635   $1,137.96   $1,137.95   $1,137.90