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 $28.54   3.225%   $29.4604   $29.46   $29.45   $29.50  
  Purchase #2 $17.33   13.762%   $19.715   $19.72   $19.70   $19.70  
  Purchase #3 $14.80   4.831%   $15.515   $15.52   $15.50   $15.50  
  Purchase #4 $135.99   9.590%   $149.0314   $149.03   $149.05   $149.00  
  Purchase #5 $139.80   8.741%   $152.0199   $152.02   $152.00   $152.00  
  Purchase #6 $35.89   8.178%   $38.8251   $38.83   $38.85   $38.80  
  Purchase #7 $147.53   8.562%   $160.1615   $160.16   $160.15   $160.20  
  7 purchase totals = $519.88       $564.7283   $564.74   $564.70   $564.70