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 $228.27   7.644%   $245.719   $245.72   $245.70   $245.70  
  Purchase #2 $66.48   11.208%   $73.9311   $73.93   $73.95   $73.90  
  Purchase #3 $73.92   5.440%   $77.9412   $77.94   $77.95   $77.90  
  Purchase #4 $125.94   8.766%   $136.9799   $136.98   $137.00   $137.00  
  Purchase #5 $138.99   4.425%   $145.1403   $145.14   $145.15   $145.10  
  Purchase #6 $95.90   6.165%   $101.8122   $101.81   $101.80   $101.80  
  Purchase #7 $54.62   6.206%   $58.0097   $58.01   $58.00   $58.00  
  7 purchase totals = $784.12       $839.5334   $839.53   $839.55   $839.40