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 $17.50   7.659%   $18.8403   $18.84   $18.85   $18.80  
  Purchase #2 $211.33   4.720%   $221.3048   $221.30   $221.30   $221.30  
  Purchase #3 $110.66   8.708%   $120.2963   $120.30   $120.30   $120.30  
  Purchase #4 $99.90   5.752%   $105.6462   $105.65   $105.65   $105.60  
  Purchase #5 $137.84   5.207%   $145.0173   $145.02   $145.00   $145.00  
  Purchase #6 $26.59   10.735%   $29.4444   $29.44   $29.45   $29.40  
  Purchase #7 $27.75   6.241%   $29.4819   $29.48   $29.50   $29.50  
  7 purchase totals = $631.57       $670.0312   $670.03   $670.05   $669.90