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 $103.89   5.117%   $109.2061   $109.21   $109.20   $109.20  
  Purchase #2 $77.36   11.285%   $86.0901   $86.09   $86.10   $86.10  
  Purchase #3 $125.31   9.592%   $137.3297   $137.33   $137.35   $137.30  
  Purchase #4 $120.57   13.521%   $136.8723   $136.87   $136.85   $136.90  
  Purchase #5 $21.57   7.283%   $23.1409   $23.14   $23.15   $23.10  
  Purchase #6 $218.50   7.585%   $235.0732   $235.07   $235.05   $235.10  
  Purchase #7 $59.30   3.477%   $61.3619   $61.36   $61.35   $61.40  
  7 purchase totals = $726.50       $789.0742   $789.07   $789.05   $789.10