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 $119.90   12.429%   $134.8024   $134.80   $134.80   $134.80  
  Purchase #2 $30.72   11.734%   $34.3247   $34.32   $34.30   $34.30  
  Purchase #3 $65.31   5.611%   $68.9745   $68.97   $68.95   $69.00  
  Purchase #4 $25.92   13.345%   $29.379   $29.38   $29.40   $29.40  
  Purchase #5 $181.41   6.216%   $192.6864   $192.69   $192.70   $192.70  
  Purchase #6 $84.72   13.364%   $96.042   $96.04   $96.05   $96.00  
  Purchase #7 $108.84   6.549%   $115.9679   $115.97   $115.95   $116.00  
  7 purchase totals = $616.82       $672.1769   $672.17   $672.15   $672.20