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 $21.27   5.566%   $22.4539   $22.45   $22.45   $22.50  
  Purchase #2 $82.76   9.356%   $90.503   $90.50   $90.50   $90.50  
  Purchase #3 $244.47   11.117%   $271.6477   $271.65   $271.65   $271.60  
  Purchase #4 $154.47   8.428%   $167.4887   $167.49   $167.50   $167.50  
  Purchase #5 $175.87   9.282%   $192.1943   $192.19   $192.20   $192.20  
  Purchase #6 $13.64   10.582%   $15.0834   $15.08   $15.10   $15.10  
  Purchase #7 $79.73   4.364%   $83.2094   $83.21   $83.20   $83.20  
  7 purchase totals = $772.21       $842.5804   $842.57   $842.60   $842.60