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 $250.73   11.651%   $279.9426   $279.94   $279.95   $279.90  
  Purchase #2 $192.53   12.682%   $216.9467   $216.95   $216.95   $216.90  
  Purchase #3 $113.34   7.572%   $121.9221   $121.92   $121.90   $121.90  
  Purchase #4 $80.66   5.246%   $84.8914   $84.89   $84.90   $84.90  
  Purchase #5 $209.81   10.216%   $231.2442   $231.24   $231.25   $231.20  
  Purchase #6 $98.51   8.570%   $106.9523   $106.95   $106.95   $107.00  
  Purchase #7 $9.33   11.594%   $10.4117   $10.41   $10.40   $10.40  
  7 purchase totals = $954.91       $1,052.3110   $1,052.30   $1,052.30   $1,052.20