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 $155.45   3.204%   $160.4306   $160.43   $160.45   $160.40  
  Purchase #2 $30.48   3.320%   $31.4919   $31.49   $31.50   $31.50  
  Purchase #3 $51.93   3.397%   $53.6941   $53.69   $53.70   $53.70  
  Purchase #4 $18.61   8.601%   $20.2106   $20.21   $20.20   $20.20  
  Purchase #5 $107.54   11.796%   $120.2254   $120.23   $120.25   $120.20  
  Purchase #6 $6.51   3.758%   $6.7546   $6.75   $6.75   $6.80  
  Purchase #7 $78.39   12.406%   $88.1151   $88.12   $88.10   $88.10  
  7 purchase totals = $448.91       $480.9223   $480.92   $480.95   $480.90