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 $246.34   13.326%   $279.1673   $279.17   $279.15   $279.20  
  Purchase #2 $86.64   12.267%   $97.2681   $97.27   $97.25   $97.30  
  Purchase #3 $70.45   12.119%   $78.9878   $78.99   $79.00   $79.00  
  Purchase #4 $244.34   11.761%   $273.0768   $273.08   $273.10   $273.10  
  Purchase #5 $153.95   5.747%   $162.7975   $162.80   $162.80   $162.80  
  Purchase #6 $60.55   13.711%   $68.852   $68.85   $68.85   $68.90  
  Purchase #7 $46.26   12.166%   $51.888   $51.89   $51.90   $51.90  
  7 purchase totals = $908.53       $1,012.0375   $1,012.05   $1,012.05   $1,012.20