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 $62.48   6.127%   $66.3081   $66.31   $66.30   $66.30  
  Purchase #2 $63.92   12.512%   $71.9177   $71.92   $71.90   $71.90  
  Purchase #3 $163.70   13.711%   $186.1449   $186.14   $186.15   $186.10  
  Purchase #4 $59.85   7.746%   $64.486   $64.49   $64.50   $64.50  
  Purchase #5 $86.66   8.505%   $94.0304   $94.03   $94.05   $94.00  
  Purchase #6 $153.85   7.859%   $165.9411   $165.94   $165.95   $165.90  
  Purchase #7 $14.87   10.568%   $16.4415   $16.44   $16.45   $16.40  
  7 purchase totals = $605.33       $665.2697   $665.27   $665.30   $665.10