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 $51.68   12.296%   $58.0346   $58.03   $58.05   $58.00  
  Purchase #2 $234.74   11.755%   $262.3337   $262.33   $262.35   $262.30  
  Purchase #3 $23.74   12.173%   $26.6299   $26.63   $26.65   $26.60  
  Purchase #4 $135.55   11.749%   $151.4758   $151.48   $151.50   $151.50  
  Purchase #5 $19.48   5.398%   $20.5315   $20.53   $20.55   $20.50  
  Purchase #6 $249.76   12.266%   $280.3956   $280.40   $280.40   $280.40  
  Purchase #7 $119.38   8.458%   $129.4772   $129.48   $129.50   $129.50  
  7 purchase totals = $834.33       $928.8783   $928.88   $929.00   $928.80