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 $249.39   10.243%   $274.935   $274.94   $274.95   $274.90  
  Purchase #2 $17.96   7.194%   $19.252   $19.25   $19.25   $19.30  
  Purchase #3 $86.44   8.643%   $93.911   $93.91   $93.90   $93.90  
  Purchase #4 $113.56   6.687%   $121.1538   $121.15   $121.15   $121.20  
  Purchase #5 $53.36   12.607%   $60.0871   $60.09   $60.10   $60.10  
  Purchase #6 $153.95   13.409%   $174.5932   $174.59   $174.60   $174.60  
  Purchase #7 $239.92   8.371%   $260.0037   $260.00   $260.00   $260.00  
  7 purchase totals = $914.58       $1,003.9358   $1,003.93   $1,003.95   $1,004.00