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 $19.70   8.458%   $21.3662   $21.37   $21.35   $21.40  
  Purchase #2 $28.68   10.515%   $31.6957   $31.70   $31.70   $31.70  
  Purchase #3 $192.86   11.372%   $214.792   $214.79   $214.80   $214.80  
  Purchase #4 $124.43   4.527%   $130.0629   $130.06   $130.05   $130.10  
  Purchase #5 $90.67   10.470%   $100.1631   $100.16   $100.15   $100.20  
  Purchase #6 $206.62   5.119%   $217.1969   $217.20   $217.20   $217.20  
  Purchase #7 $25.99   12.775%   $29.3102   $29.31   $29.30   $29.30  
  7 purchase totals = $688.95       $744.5870   $744.59   $744.55   $744.70