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 $16.90   6.422%   $17.9853   $17.99   $18.00   $18.00  
  Purchase #2 $140.55   8.812%   $152.9353   $152.94   $152.95   $152.90  
  Purchase #3 $147.88   3.169%   $152.5663   $152.57   $152.55   $152.60  
  Purchase #4 $50.55   10.202%   $55.7071   $55.71   $55.70   $55.70  
  Purchase #5 $241.50   3.829%   $250.747   $250.75   $250.75   $250.70  
  Purchase #6 $218.87   12.871%   $247.0408   $247.04   $247.05   $247.00  
  Purchase #7 $94.52   9.127%   $103.1468   $103.15   $103.15   $103.10  
  7 purchase totals = $910.77       $980.1286   $980.15   $980.15   $980.00