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 $246.94   10.860%   $273.7577   $273.76   $273.75   $273.80  
  Purchase #2 $160.82   7.108%   $172.2511   $172.25   $172.25   $172.30  
  Purchase #3 $222.69   9.209%   $243.1975   $243.20   $243.20   $243.20  
  Purchase #4 $101.69   11.422%   $113.305   $113.31   $113.30   $113.30  
  Purchase #5 $136.45   5.767%   $144.3191   $144.32   $144.30   $144.30  
  Purchase #6 $42.94   9.551%   $47.0412   $47.04   $47.05   $47.00  
  Purchase #7 $230.90   10.865%   $255.9873   $255.99   $256.00   $256.00  
  7 purchase totals = $1,142.43       $1,249.8589   $1,249.87   $1,249.85   $1,249.90