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 $250.30   9.397%   $273.8207   $273.82   $273.80   $273.80  
  Purchase #2 $79.74   3.602%   $82.6122   $82.61   $82.60   $82.60  
  Purchase #3 $80.70   7.438%   $86.7025   $86.70   $86.70   $86.70  
  Purchase #4 $228.43   11.659%   $255.0627   $255.06   $255.05   $255.10  
  Purchase #5 $235.64   6.241%   $250.3463   $250.35   $250.35   $250.30  
  Purchase #6 $236.85   4.284%   $246.9967   $247.00   $247.00   $247.00  
  Purchase #7 $151.39   7.721%   $163.0788   $163.08   $163.10   $163.10  
  7 purchase totals = $1,263.05       $1,358.6199   $1,358.62   $1,358.60   $1,358.60