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 $234.30   11.370%   $260.9399   $260.94   $260.95   $260.90  
  Purchase #2 $176.26   9.744%   $193.4348   $193.43   $193.45   $193.40  
  Purchase #3 $55.30   9.591%   $60.6038   $60.60   $60.60   $60.60  
  Purchase #4 $159.76   5.564%   $168.649   $168.65   $168.65   $168.60  
  Purchase #5 $78.74   3.860%   $81.7794   $81.78   $81.80   $81.80  
  Purchase #6 $165.92   9.683%   $181.986   $181.99   $182.00   $182.00  
  Purchase #7 $45.83   11.705%   $51.1944   $51.19   $51.20   $51.20  
  7 purchase totals = $916.11       $998.5873   $998.58   $998.65   $998.50