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 $124.44   12.839%   $140.4169   $140.42   $140.40   $140.40  
  Purchase #2 $208.40   5.766%   $220.4163   $220.42   $220.40   $220.40  
  Purchase #3 $26.94   6.427%   $28.6714   $28.67   $28.65   $28.70  
  Purchase #4 $7.99   4.549%   $8.3535   $8.35   $8.35   $8.40  
  Purchase #5 $107.32   7.283%   $115.1361   $115.14   $115.15   $115.10  
  Purchase #6 $105.53   13.209%   $119.4695   $119.47   $119.45   $119.50  
  Purchase #7 $201.84   8.168%   $218.3263   $218.33   $218.35   $218.30  
  7 purchase totals = $782.46       $850.7900   $850.80   $850.75   $850.80