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 $230.25   4.449%   $240.4938   $240.49   $240.50   $240.50  
  Purchase #2 $122.82   5.317%   $129.3503   $129.35   $129.35   $129.40  
  Purchase #3 $213.59   4.854%   $223.9577   $223.96   $223.95   $224.00  
  Purchase #4 $83.48   7.836%   $90.0215   $90.02   $90.00   $90.00  
  Purchase #5 $20.37   13.349%   $23.0892   $23.09   $23.10   $23.10  
  Purchase #6 $8.40   8.839%   $9.1425   $9.14   $9.15   $9.10  
  Purchase #7 $153.59   6.752%   $163.9604   $163.96   $163.95   $164.00  
  7 purchase totals = $832.50       $880.0154   $880.01   $880.00   $880.10