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 $29.90   3.401%   $30.9169   $30.92   $30.90   $30.90  
  Purchase #2 $127.40   8.154%   $137.7882   $137.79   $137.80   $137.80  
  Purchase #3 $97.52   4.325%   $101.7377   $101.74   $101.75   $101.70  
  Purchase #4 $108.57   7.313%   $116.5097   $116.51   $116.50   $116.50  
  Purchase #5 $53.63   7.270%   $57.5289   $57.53   $57.55   $57.50  
  Purchase #6 $29.58   3.524%   $30.6224   $30.62   $30.60   $30.60  
  Purchase #7 $193.93   5.480%   $204.5574   $204.56   $204.55   $204.60  
  7 purchase totals = $640.53       $679.6612   $679.67   $679.65   $679.60