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 $242.96   3.105%   $250.5039   $250.50   $250.50   $250.50  
  Purchase #2 $31.65   6.703%   $33.7715   $33.77   $33.75   $33.80  
  Purchase #3 $195.88   3.216%   $202.1795   $202.18   $202.20   $202.20  
  Purchase #4 $189.89   3.271%   $196.1013   $196.10   $196.10   $196.10  
  Purchase #5 $52.50   13.452%   $59.5623   $59.56   $59.55   $59.60  
  Purchase #6 $5.61   7.456%   $6.0283   $6.03   $6.05   $6.00  
  Purchase #7 $80.74   4.491%   $84.366   $84.37   $84.35   $84.40  
  7 purchase totals = $799.23       $832.5128   $832.51   $832.50   $832.60