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 $18.40   8.781%   $20.0157   $20.02   $20.00   $20.00  
  Purchase #2 $187.51   12.730%   $211.38   $211.38   $211.40   $211.40  
  Purchase #3 $121.26   10.706%   $134.2421   $134.24   $134.25   $134.20  
  Purchase #4 $35.47   7.589%   $38.1618   $38.16   $38.15   $38.20  
  Purchase #5 $8.96   12.124%   $10.0463   $10.05   $10.05   $10.00  
  Purchase #6 $64.55   8.421%   $69.9858   $69.99   $70.00   $70.00  
  Purchase #7 $77.72   4.210%   $80.992   $80.99   $81.00   $81.00  
  7 purchase totals = $513.87       $564.8237   $564.83   $564.85   $564.80