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 $17.78   4.320%   $18.5481   $18.55   $18.55   $18.50  
  Purchase #2 $37.81   11.468%   $42.1461   $42.15   $42.15   $42.10  
  Purchase #3 $19.71   11.730%   $22.022   $22.02   $22.00   $22.00  
  Purchase #4 $199.54   10.563%   $220.6174   $220.62   $220.60   $220.60  
  Purchase #5 $28.94   4.175%   $30.1482   $30.15   $30.15   $30.10  
  Purchase #6 $60.70   3.125%   $62.5969   $62.60   $62.60   $62.60  
  Purchase #7 $152.32   13.782%   $173.3127   $173.31   $173.30   $173.30  
  7 purchase totals = $516.80       $569.3914   $569.40   $569.35   $569.20