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 $25.58   13.452%   $29.021   $29.02   $29.00   $29.00  
  Purchase #2 $218.97   9.873%   $240.5889   $240.59   $240.60   $240.60  
  Purchase #3 $50.80   9.351%   $55.5503   $55.55   $55.55   $55.60  
  Purchase #4 $109.74   4.410%   $114.5795   $114.58   $114.60   $114.60  
  Purchase #5 $68.71   6.589%   $73.2373   $73.24   $73.25   $73.20  
  Purchase #6 $44.52   10.110%   $49.021   $49.02   $49.00   $49.00  
  Purchase #7 $60.70   11.580%   $67.7291   $67.73   $67.75   $67.70  
  7 purchase totals = $579.02       $629.7271   $629.73   $629.75   $629.70