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 $246.47   13.377%   $279.4403   $279.44   $279.45   $279.40  
  Purchase #2 $165.38   6.371%   $175.9164   $175.92   $175.90   $175.90  
  Purchase #3 $69.28   13.627%   $78.7208   $78.72   $78.70   $78.70  
  Purchase #4 $239.89   8.213%   $259.5922   $259.59   $259.60   $259.60  
  Purchase #5 $198.84   5.130%   $209.0405   $209.04   $209.05   $209.00  
  Purchase #6 $74.81   10.326%   $82.5349   $82.53   $82.55   $82.50  
  Purchase #7 $69.55   6.400%   $74.0012   $74.00   $74.00   $74.00  
  7 purchase totals = $1,064.22       $1,159.2463   $1,159.24   $1,159.25   $1,159.10