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 $66.55   3.205%   $68.6829   $68.68   $68.70   $68.70  
  Purchase #2 $156.93   12.240%   $176.1382   $176.14   $176.15   $176.10  
  Purchase #3 $113.88   4.705%   $119.2381   $119.24   $119.25   $119.20  
  Purchase #4 $12.97   11.798%   $14.5002   $14.50   $14.50   $14.50  
  Purchase #5 $11.60   6.188%   $12.3178   $12.32   $12.30   $12.30  
  Purchase #6 $249.86   11.849%   $279.4659   $279.47   $279.45   $279.50  
  Purchase #7 $180.88   3.349%   $186.9377   $186.94   $186.95   $186.90  
  7 purchase totals = $792.67       $857.2808   $857.29   $857.30   $857.20