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 $15.70   13.301%   $17.7883   $17.79   $17.80   $17.80  
  Purchase #2 $13.91   7.827%   $14.9987   $15.00   $15.00   $15.00  
  Purchase #3 $150.87   13.260%   $170.8754   $170.88   $170.90   $170.90  
  Purchase #4 $222.33   4.367%   $232.0392   $232.04   $232.05   $232.00  
  Purchase #5 $228.40   6.633%   $243.5498   $243.55   $243.55   $243.50  
  Purchase #6 $109.34   12.159%   $122.6347   $122.63   $122.65   $122.60  
  Purchase #7 $10.27   3.139%   $10.5924   $10.59   $10.60   $10.60  
  7 purchase totals = $750.82       $812.4785   $812.48   $812.55   $812.40