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 $243.78   10.156%   $268.5383   $268.54   $268.55   $268.50  
  Purchase #2 $191.59   9.496%   $209.7834   $209.78   $209.80   $209.80  
  Purchase #3 $25.74   9.753%   $28.2504   $28.25   $28.25   $28.30  
  Purchase #4 $12.95   8.242%   $14.0173   $14.02   $14.00   $14.00  
  Purchase #5 $114.87   3.309%   $118.671   $118.67   $118.65   $118.70  
  Purchase #6 $97.64   7.535%   $104.9972   $105.00   $105.00   $105.00  
  Purchase #7 $222.47   7.686%   $239.569   $239.57   $239.55   $239.60  
  7 purchase totals = $909.04       $983.8266   $983.83   $983.80   $983.90