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 $238.81   6.367%   $254.015   $254.02   $254.00   $254.00  
  Purchase #2 $112.75   6.127%   $119.6582   $119.66   $119.65   $119.70  
  Purchase #3 $228.74   12.559%   $257.4675   $257.47   $257.45   $257.50  
  Purchase #4 $121.98   7.461%   $131.0809   $131.08   $131.10   $131.10  
  Purchase #5 $37.98   12.309%   $42.655   $42.66   $42.65   $42.70  
  Purchase #6 $239.50   5.183%   $251.9133   $251.91   $251.90   $251.90  
  Purchase #7 $203.33   13.851%   $231.4932   $231.49   $231.50   $231.50  
  7 purchase totals = $1,183.09       $1,288.2831   $1,288.29   $1,288.25   $1,288.40