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 $163.88   9.854%   $180.0287   $180.03   $180.05   $180.00  
  Purchase #2 $19.94   3.547%   $20.6473   $20.65   $20.65   $20.60  
  Purchase #3 $170.79   5.450%   $180.0981   $180.10   $180.10   $180.10  
  Purchase #4 $16.72   4.645%   $17.4966   $17.50   $17.50   $17.50  
  Purchase #5 $232.75   9.730%   $255.3966   $255.40   $255.40   $255.40  
  Purchase #6 $13.37   13.599%   $15.1882   $15.19   $15.20   $15.20  
  Purchase #7 $192.86   12.409%   $216.792   $216.79   $216.80   $216.80  
  7 purchase totals = $810.31       $885.6475   $885.66   $885.70   $885.60