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 $19.48   3.435%   $20.1491   $20.15   $20.15   $20.10  
  Purchase #2 $126.55   9.349%   $138.3812   $138.38   $138.40   $138.40  
  Purchase #3 $22.40   5.552%   $23.6436   $23.64   $23.65   $23.60  
  Purchase #4 $66.25   10.315%   $73.0837   $73.08   $73.10   $73.10  
  Purchase #5 $46.47   13.720%   $52.8457   $52.85   $52.85   $52.80  
  Purchase #6 $166.85   7.646%   $179.6074   $179.61   $179.60   $179.60  
  Purchase #7 $181.98   3.308%   $187.9999   $188.00   $188.00   $188.00  
  7 purchase totals = $629.98       $675.7106   $675.71   $675.75   $675.60