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 $28.49   6.744%   $30.4114   $30.41   $30.40   $30.40  
  Purchase #2 $72.84   5.846%   $77.0982   $77.10   $77.10   $77.10  
  Purchase #3 $44.75   10.602%   $49.4944   $49.49   $49.50   $49.50  
  Purchase #4 $63.88   10.485%   $70.5778   $70.58   $70.60   $70.60  
  Purchase #5 $28.94   9.778%   $31.7698   $31.77   $31.75   $31.80  
  Purchase #6 $151.95   13.593%   $172.6046   $172.60   $172.60   $172.60  
  Purchase #7 $99.62   5.826%   $105.4239   $105.42   $105.40   $105.40  
  7 purchase totals = $490.47       $537.3801   $537.37   $537.35   $537.40