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 $250.83   6.752%   $267.766   $267.77   $267.75   $267.80  
  Purchase #2 $90.62   3.749%   $94.0173   $94.02   $94.00   $94.00  
  Purchase #3 $58.94   6.490%   $62.7652   $62.77   $62.75   $62.80  
  Purchase #4 $151.48   8.576%   $164.4709   $164.47   $164.45   $164.50  
  Purchase #5 $101.97   6.742%   $108.8448   $108.84   $108.85   $108.80  
  Purchase #6 $130.53   7.703%   $140.5847   $140.58   $140.60   $140.60  
  Purchase #7 $139.46   4.190%   $145.3034   $145.30   $145.30   $145.30  
  7 purchase totals = $923.83       $983.7523   $983.75   $983.70   $983.80