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 $248.62   10.125%   $273.7928   $273.79   $273.80   $273.80  
  Purchase #2 $154.79   12.716%   $174.4731   $174.47   $174.45   $174.50  
  Purchase #3 $231.66   11.575%   $258.4746   $258.47   $258.45   $258.50  
  Purchase #4 $66.95   13.185%   $75.7774   $75.78   $75.80   $75.80  
  Purchase #5 $79.76   5.549%   $84.1859   $84.19   $84.20   $84.20  
  Purchase #6 $97.98   6.637%   $104.4829   $104.48   $104.50   $104.50  
  Purchase #7 $46.62   7.459%   $50.0974   $50.10   $50.10   $50.10  
  7 purchase totals = $926.38       $1,021.2841   $1,021.28   $1,021.30   $1,021.40