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 $222.80   12.568%   $250.8015   $250.80   $250.80   $250.80  
  Purchase #2 $83.99   13.474%   $95.3068   $95.31   $95.30   $95.30  
  Purchase #3 $59.46   5.133%   $62.5121   $62.51   $62.50   $62.50  
  Purchase #4 $41.86   6.373%   $44.5277   $44.53   $44.55   $44.50  
  Purchase #5 $248.96   9.820%   $273.4079   $273.41   $273.40   $273.40  
  Purchase #6 $61.30   3.783%   $63.619   $63.62   $63.60   $63.60  
  Purchase #7 $8.64   12.587%   $9.7275   $9.73   $9.75   $9.70  
  7 purchase totals = $727.01       $799.9025   $799.91   $799.90   $799.80