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 $100.51   13.256%   $113.8336   $113.83   $113.85   $113.80  
  Purchase #2 $92.28   10.749%   $102.1992   $102.20   $102.20   $102.20  
  Purchase #3 $47.55   11.214%   $52.8823   $52.88   $52.90   $52.90  
  Purchase #4 $122.95   3.802%   $127.6246   $127.62   $127.60   $127.60  
  Purchase #5 $237.63   8.504%   $257.8381   $257.84   $257.85   $257.80  
  Purchase #6 $18.64   12.743%   $21.0153   $21.02   $21.00   $21.00  
  Purchase #7 $33.94   13.315%   $38.4591   $38.46   $38.45   $38.50  
  7 purchase totals = $653.50       $713.8522   $713.85   $713.85   $713.80