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 $131.59   13.831%   $149.7902   $149.79   $149.80   $149.80  
  Purchase #2 $227.33   7.150%   $243.5841   $243.58   $243.60   $243.60  
  Purchase #3 $105.49   10.544%   $116.6129   $116.61   $116.60   $116.60  
  Purchase #4 $188.79   5.343%   $198.877   $198.88   $198.90   $198.90  
  Purchase #5 $20.42   4.186%   $21.2748   $21.27   $21.25   $21.30  
  Purchase #6 $51.44   7.644%   $55.3721   $55.37   $55.35   $55.40  
  Purchase #7 $26.92   4.516%   $28.1357   $28.14   $28.15   $28.10  
  7 purchase totals = $751.98       $813.6468   $813.64   $813.65   $813.70