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 $223.35   11.498%   $249.0308   $249.03   $249.05   $249.00  
  Purchase #2 $115.57   11.599%   $128.975   $128.98   $129.00   $129.00  
  Purchase #3 $31.59   10.866%   $35.0226   $35.02   $35.00   $35.00  
  Purchase #4 $46.39   7.609%   $49.9198   $49.92   $49.90   $49.90  
  Purchase #5 $59.45   8.273%   $64.3683   $64.37   $64.35   $64.40  
  Purchase #6 $163.85   8.526%   $177.8199   $177.82   $177.80   $177.80  
  Purchase #7 $54.77   8.806%   $59.593   $59.59   $59.60   $59.60  
  7 purchase totals = $694.97       $764.7294   $764.73   $764.70   $764.70