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 $58.78   12.822%   $66.3168   $66.32   $66.30   $66.30  
  Purchase #2 $244.87   5.310%   $257.8726   $257.87   $257.85   $257.90  
  Purchase #3 $98.31   12.144%   $110.2488   $110.25   $110.25   $110.20  
  Purchase #4 $187.78   9.751%   $206.0904   $206.09   $206.10   $206.10  
  Purchase #5 $14.63   6.690%   $15.6087   $15.61   $15.60   $15.60  
  Purchase #6 $105.34   5.294%   $110.9167   $110.92   $110.90   $110.90  
  Purchase #7 $8.74   6.476%   $9.306   $9.31   $9.30   $9.30  
  7 purchase totals = $718.45       $776.3600   $776.37   $776.30   $776.30