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 $11.87   12.729%   $13.3809   $13.38   $13.40   $13.40  
  Purchase #2 $241.68   6.603%   $257.6381   $257.64   $257.65   $257.60  
  Purchase #3 $165.35   3.177%   $170.6032   $170.60   $170.60   $170.60  
  Purchase #4 $218.75   12.773%   $246.6909   $246.69   $246.70   $246.70  
  Purchase #5 $26.55   3.725%   $27.539   $27.54   $27.55   $27.50  
  Purchase #6 $65.28   6.386%   $69.4488   $69.45   $69.45   $69.40  
  Purchase #7 $163.92   7.232%   $175.7747   $175.77   $175.75   $175.80  
  7 purchase totals = $893.40       $961.0756   $961.07   $961.10   $961.00