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 $11.80   9.260%   $12.8927   $12.89   $12.90   $12.90  
  Purchase #2 $15.66   13.499%   $17.7739   $17.77   $17.75   $17.80  
  Purchase #3 $53.61   4.169%   $55.845   $55.85   $55.85   $55.80  
  Purchase #4 $164.50   13.492%   $186.6943   $186.69   $186.70   $186.70  
  Purchase #5 $189.72   10.431%   $209.5097   $209.51   $209.50   $209.50  
  Purchase #6 $76.86   11.507%   $85.7043   $85.70   $85.70   $85.70  
  Purchase #7 $12.88   6.701%   $13.7431   $13.74   $13.75   $13.70  
  7 purchase totals = $525.03       $582.1630   $582.15   $582.15   $582.10