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 $121.74   3.861%   $126.4404   $126.44   $126.45   $126.40  
  Purchase #2 $65.78   3.415%   $68.0264   $68.03   $68.05   $68.00  
  Purchase #3 $63.76   3.493%   $65.9871   $65.99   $66.00   $66.00  
  Purchase #4 $240.67   8.783%   $261.808   $261.81   $261.80   $261.80  
  Purchase #5 $33.32   7.336%   $35.7644   $35.76   $35.75   $35.80  
  Purchase #6 $29.76   8.791%   $32.3762   $32.38   $32.40   $32.40  
  Purchase #7 $82.90   6.205%   $88.0439   $88.04   $88.05   $88.00  
  7 purchase totals = $637.93       $678.4464   $678.45   $678.50   $678.40