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 $39.58   6.539%   $42.1681   $42.17   $42.15   $42.20  
  Purchase #2 $136.95   8.221%   $148.2087   $148.21   $148.20   $148.20  
  Purchase #3 $15.47   9.560%   $16.9489   $16.95   $16.95   $16.90  
  Purchase #4 $131.64   10.131%   $144.9764   $144.98   $145.00   $145.00  
  Purchase #5 $28.64   13.809%   $32.5949   $32.59   $32.60   $32.60  
  Purchase #6 $123.43   5.594%   $130.3347   $130.33   $130.35   $130.30  
  Purchase #7 $111.74   13.225%   $126.5176   $126.52   $126.50   $126.50  
  7 purchase totals = $587.45       $641.7493   $641.75   $641.75   $641.70