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 $12.90   6.279%   $13.71   $13.71   $13.70   $13.70  
  Purchase #2 $55.56   3.618%   $57.5702   $57.57   $57.55   $57.60  
  Purchase #3 $244.32   8.670%   $265.5025   $265.50   $265.50   $265.50  
  Purchase #4 $167.42   5.803%   $177.1354   $177.14   $177.15   $177.10  
  Purchase #5 $239.68   6.416%   $255.0579   $255.06   $255.05   $255.10  
  Purchase #6 $125.40   11.672%   $140.0367   $140.04   $140.05   $140.00  
  Purchase #7 $145.71   5.318%   $153.4589   $153.46   $153.45   $153.50  
  7 purchase totals = $990.99       $1,062.4716   $1,062.48   $1,062.45   $1,062.50