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 $220.74   13.823%   $251.2529   $251.25   $251.25   $251.30  
  Purchase #2 $179.31   5.363%   $188.9264   $188.93   $188.95   $188.90  
  Purchase #3 $15.55   5.173%   $16.3544   $16.35   $16.35   $16.40  
  Purchase #4 $88.26   10.763%   $97.7594   $97.76   $97.75   $97.80  
  Purchase #5 $116.53   7.724%   $125.5308   $125.53   $125.55   $125.50  
  Purchase #6 $134.90   9.387%   $147.5631   $147.56   $147.55   $147.60  
  Purchase #7 $112.94   7.613%   $121.5381   $121.54   $121.55   $121.50  
  7 purchase totals = $868.23       $948.9251   $948.92   $948.95   $949.00