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 $48.73   5.683%   $51.4993   $51.50   $51.50   $51.50  
  Purchase #2 $240.80   11.854%   $269.3444   $269.34   $269.35   $269.30  
  Purchase #3 $180.27   7.787%   $194.3076   $194.31   $194.30   $194.30  
  Purchase #4 $205.63   8.786%   $223.6967   $223.70   $223.70   $223.70  
  Purchase #5 $24.74   10.716%   $27.3911   $27.39   $27.40   $27.40  
  Purchase #6 $16.85   4.836%   $17.6649   $17.66   $17.65   $17.70  
  Purchase #7 $209.86   6.689%   $223.8975   $223.90   $223.90   $223.90  
  7 purchase totals = $926.88       $1,007.8015   $1,007.80   $1,007.80   $1,007.80