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 $232.79   13.616%   $264.4867   $264.49   $264.50   $264.50  
  Purchase #2 $102.72   10.170%   $113.1666   $113.17   $113.15   $113.20  
  Purchase #3 $35.81   12.701%   $40.3582   $40.36   $40.35   $40.40  
  Purchase #4 $208.65   13.120%   $236.0249   $236.02   $236.00   $236.00  
  Purchase #5 $147.88   5.822%   $156.4896   $156.49   $156.50   $156.50  
  Purchase #6 $162.51   3.532%   $168.2499   $168.25   $168.25   $168.20  
  Purchase #7 $11.96   7.806%   $12.8936   $12.89   $12.90   $12.90  
  7 purchase totals = $902.32       $991.6695   $991.67   $991.65   $991.70