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 $226.27   13.586%   $257.011   $257.01   $257.00   $257.00  
  Purchase #2 $110.75   12.839%   $124.9692   $124.97   $124.95   $125.00  
  Purchase #3 $129.91   7.695%   $139.9066   $139.91   $139.90   $139.90  
  Purchase #4 $196.41   7.289%   $210.7263   $210.73   $210.75   $210.70  
  Purchase #5 $73.75   9.116%   $80.4731   $80.47   $80.45   $80.50  
  Purchase #6 $105.97   5.844%   $112.1629   $112.16   $112.15   $112.20  
  Purchase #7 $153.49   9.492%   $168.0593   $168.06   $168.05   $168.10  
  7 purchase totals = $996.55       $1,093.3084   $1,093.31   $1,093.25   $1,093.40