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 $247.95   4.447%   $258.9763   $258.98   $259.00   $259.00  
  Purchase #2 $137.70   11.656%   $153.7503   $153.75   $153.75   $153.80  
  Purchase #3 $178.92   11.635%   $199.7373   $199.74   $199.75   $199.70  
  Purchase #4 $124.48   12.228%   $139.7014   $139.70   $139.70   $139.70  
  Purchase #5 $51.31   8.349%   $55.5939   $55.59   $55.60   $55.60  
  Purchase #6 $225.75   3.543%   $233.7483   $233.75   $233.75   $233.70  
  Purchase #7 $109.32   8.455%   $118.563   $118.56   $118.55   $118.60  
  7 purchase totals = $1,075.43       $1,160.0705   $1,160.07   $1,160.10   $1,160.10