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 $83.25   8.309%   $90.1672   $90.17   $90.15   $90.20  
  Purchase #2 $98.26   10.186%   $108.2688   $108.27   $108.25   $108.30  
  Purchase #3 $71.80   5.800%   $75.9644   $75.96   $75.95   $76.00  
  Purchase #4 $173.72   10.167%   $191.3821   $191.38   $191.40   $191.40  
  Purchase #5 $73.67   3.669%   $76.373   $76.37   $76.35   $76.40  
  Purchase #6 $80.76   10.334%   $89.1057   $89.11   $89.10   $89.10  
  Purchase #7 $44.49   11.662%   $49.6784   $49.68   $49.70   $49.70  
  7 purchase totals = $625.95       $680.9396   $680.94   $680.90   $681.10