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 $246.56   12.174%   $276.5762   $276.58   $276.60   $276.60  
  Purchase #2 $233.53   4.734%   $244.5853   $244.59   $244.60   $244.60  
  Purchase #3 $50.56   4.184%   $52.6754   $52.68   $52.70   $52.70  
  Purchase #4 $206.37   7.106%   $221.0347   $221.03   $221.05   $221.00  
  Purchase #5 $63.96   3.865%   $66.4321   $66.43   $66.45   $66.40  
  Purchase #6 $121.75   10.565%   $134.6129   $134.61   $134.60   $134.60  
  Purchase #7 $23.43   7.749%   $25.2456   $25.25   $25.25   $25.20  
  7 purchase totals = $946.16       $1,021.1622   $1,021.17   $1,021.25   $1,021.10