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 $28.93   4.357%   $30.1905   $30.19   $30.20   $30.20  
  Purchase #2 $45.41   12.695%   $51.1748   $51.17   $51.15   $51.20  
  Purchase #3 $14.62   5.729%   $15.4576   $15.46   $15.45   $15.50  
  Purchase #4 $167.72   12.737%   $189.0825   $189.08   $189.10   $189.10  
  Purchase #5 $55.62   9.408%   $60.8527   $60.85   $60.85   $60.90  
  Purchase #6 $175.52   8.532%   $190.4954   $190.50   $190.50   $190.50  
  Purchase #7 $20.28   4.451%   $21.1827   $21.18   $21.20   $21.20  
  7 purchase totals = $508.10       $558.4362   $558.43   $558.45   $558.60