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 $35.85   12.207%   $40.2262   $40.23   $40.25   $40.20  
  Purchase #2 $217.99   10.587%   $241.0686   $241.07   $241.05   $241.10  
  Purchase #3 $47.67   13.267%   $53.9944   $53.99   $54.00   $54.00  
  Purchase #4 $126.84   10.591%   $140.2736   $140.27   $140.25   $140.30  
  Purchase #5 $73.45   11.857%   $82.159   $82.16   $82.15   $82.20  
  Purchase #6 $156.60   9.460%   $171.4144   $171.41   $171.40   $171.40  
  Purchase #7 $130.49   12.371%   $146.6329   $146.63   $146.65   $146.60  
  7 purchase totals = $788.89       $875.7691   $875.76   $875.75   $875.80