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 $224.66   6.195%   $238.5777   $238.58   $238.60   $238.60  
  Purchase #2 $130.76   12.186%   $146.6944   $146.69   $146.70   $146.70  
  Purchase #3 $143.91   11.257%   $160.1099   $160.11   $160.10   $160.10  
  Purchase #4 $90.28   7.508%   $97.0582   $97.06   $97.05   $97.10  
  Purchase #5 $10.31   3.238%   $10.6438   $10.64   $10.65   $10.60  
  Purchase #6 $247.34   6.406%   $263.1846   $263.18   $263.20   $263.20  
  Purchase #7 $102.28   6.161%   $108.5815   $108.58   $108.60   $108.60  
  7 purchase totals = $949.54       $1,024.8501   $1,024.84   $1,024.90   $1,024.90