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 $125.60   11.592%   $140.1596   $140.16   $140.15   $140.20  
  Purchase #2 $107.79   12.235%   $120.9781   $120.98   $121.00   $121.00  
  Purchase #3 $143.28   7.578%   $154.1378   $154.14   $154.15   $154.10  
  Purchase #4 $22.58   4.125%   $23.5114   $23.51   $23.50   $23.50  
  Purchase #5 $24.40   3.453%   $25.2425   $25.24   $25.25   $25.20  
  Purchase #6 $125.70   8.236%   $136.0527   $136.05   $136.05   $136.10  
  Purchase #7 $35.65   9.509%   $39.04   $39.04   $39.05   $39.00  
  7 purchase totals = $585.00       $639.1221   $639.12   $639.15   $639.10