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 $99.93   8.442%   $108.3661   $108.37   $108.35   $108.40  
  Purchase #2 $134.70   4.641%   $140.9514   $140.95   $140.95   $141.00  
  Purchase #3 $80.35   12.148%   $90.1109   $90.11   $90.10   $90.10  
  Purchase #4 $72.70   7.812%   $78.3793   $78.38   $78.40   $78.40  
  Purchase #5 $52.73   11.323%   $58.7006   $58.70   $58.70   $58.70  
  Purchase #6 $243.48   9.476%   $266.5522   $266.55   $266.55   $266.60  
  Purchase #7 $61.70   12.169%   $69.2083   $69.21   $69.20   $69.20  
  7 purchase totals = $745.59       $812.2688   $812.27   $812.25   $812.40