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 $129.53   4.490%   $135.3459   $135.35   $135.35   $135.30  
  Purchase #2 $166.36   13.222%   $188.3561   $188.36   $188.35   $188.40  
  Purchase #3 $187.25   6.865%   $200.1047   $200.10   $200.10   $200.10  
  Purchase #4 $186.60   11.614%   $208.2717   $208.27   $208.25   $208.30  
  Purchase #5 $247.63   9.204%   $270.4219   $270.42   $270.40   $270.40  
  Purchase #6 $94.78   5.488%   $99.9815   $99.98   $100.00   $100.00  
  Purchase #7 $17.27   6.846%   $18.4523   $18.45   $18.45   $18.50  
  7 purchase totals = $1,029.42       $1,120.9341   $1,120.93   $1,120.90   $1,121.00