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 $44.36   13.175%   $50.2044   $50.20   $50.20   $50.20  
  Purchase #2 $125.57   11.655%   $140.2052   $140.21   $140.20   $140.20  
  Purchase #3 $94.39   11.409%   $105.159   $105.16   $105.15   $105.20  
  Purchase #4 $237.86   5.861%   $251.801   $251.80   $251.80   $251.80  
  Purchase #5 $179.47   11.375%   $199.8847   $199.88   $199.90   $199.90  
  Purchase #6 $16.76   10.579%   $18.533   $18.53   $18.55   $18.50  
  Purchase #7 $238.59   4.813%   $250.0733   $250.07   $250.05   $250.10  
  7 purchase totals = $937.00       $1,015.8606   $1,015.85   $1,015.85   $1,015.90