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 $19.97   4.667%   $20.902   $20.90   $20.90   $20.90  
  Purchase #2 $5.85   3.757%   $6.0698   $6.07   $6.05   $6.10  
  Purchase #3 $144.77   7.246%   $155.26   $155.26   $155.25   $155.30  
  Purchase #4 $121.83   10.662%   $134.8195   $134.82   $134.80   $134.80  
  Purchase #5 $73.87   11.772%   $82.566   $82.57   $82.55   $82.60  
  Purchase #6 $235.32   9.247%   $257.08   $257.08   $257.10   $257.10  
  Purchase #7 $228.44   3.660%   $236.8009   $236.80   $236.80   $236.80  
  7 purchase totals = $830.05       $893.4982   $893.50   $893.45   $893.60