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 $74.81   12.579%   $84.2203   $84.22   $84.20   $84.20  
  Purchase #2 $226.69   13.777%   $257.9211   $257.92   $257.90   $257.90  
  Purchase #3 $248.43   13.332%   $281.5507   $281.55   $281.55   $281.60  
  Purchase #4 $81.39   3.242%   $84.0287   $84.03   $84.05   $84.00  
  Purchase #5 $100.99   8.176%   $109.2469   $109.25   $109.25   $109.20  
  Purchase #6 $60.44   5.514%   $63.7727   $63.77   $63.75   $63.80  
  Purchase #7 $165.67   11.854%   $185.3085   $185.31   $185.30   $185.30  
  7 purchase totals = $958.42       $1,066.0489   $1,066.05   $1,066.00   $1,066.00