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 $72.54   10.110%   $79.8738   $79.87   $79.85   $79.90  
  Purchase #2 $121.66   5.691%   $128.5837   $128.58   $128.60   $128.60  
  Purchase #3 $129.63   12.736%   $146.1397   $146.14   $146.15   $146.10  
  Purchase #4 $31.29   6.145%   $33.2128   $33.21   $33.20   $33.20  
  Purchase #5 $33.41   3.108%   $34.4484   $34.45   $34.45   $34.40  
  Purchase #6 $86.30   3.507%   $89.3265   $89.33   $89.35   $89.30  
  Purchase #7 $26.42   10.537%   $29.2039   $29.20   $29.20   $29.20  
  7 purchase totals = $501.25       $540.7888   $540.78   $540.80   $540.70