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 $17.76   5.772%   $18.7851   $18.79   $18.80   $18.80  
  Purchase #2 $16.49   12.750%   $18.5925   $18.59   $18.60   $18.60  
  Purchase #3 $90.82   10.586%   $100.4342   $100.43   $100.45   $100.40  
  Purchase #4 $116.56   4.409%   $121.6991   $121.70   $121.70   $121.70  
  Purchase #5 $41.95   6.773%   $44.7913   $44.79   $44.80   $44.80  
  Purchase #6 $202.44   3.118%   $208.7521   $208.75   $208.75   $208.80  
  Purchase #7 $115.77   5.236%   $121.8317   $121.83   $121.85   $121.80  
  7 purchase totals = $601.79       $634.8860   $634.88   $634.95   $634.90