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 $123.44   11.779%   $137.98   $137.98   $138.00   $138.00  
  Purchase #2 $50.65   4.584%   $52.9718   $52.97   $52.95   $53.00  
  Purchase #3 $205.53   8.708%   $223.4276   $223.43   $223.45   $223.40  
  Purchase #4 $57.99   13.424%   $65.7746   $65.77   $65.75   $65.80  
  Purchase #5 $241.36   8.505%   $261.8877   $261.89   $261.90   $261.90  
  Purchase #6 $233.69   11.531%   $260.6368   $260.64   $260.65   $260.60  
  Purchase #7 $127.56   7.240%   $136.7953   $136.80   $136.80   $136.80  
  7 purchase totals = $1,040.22       $1,139.4738   $1,139.48   $1,139.50   $1,139.50