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 $216.55   13.710%   $246.239   $246.24   $246.25   $246.20  
  Purchase #2 $10.57   11.683%   $11.8049   $11.80   $11.80   $11.80  
  Purchase #3 $68.25   5.119%   $71.7437   $71.74   $71.75   $71.70  
  Purchase #4 $175.62   11.857%   $196.4433   $196.44   $196.45   $196.40  
  Purchase #5 $131.90   8.701%   $143.3766   $143.38   $143.40   $143.40  
  Purchase #6 $161.91   8.643%   $175.9039   $175.90   $175.90   $175.90  
  Purchase #7 $141.55   4.424%   $147.8122   $147.81   $147.80   $147.80  
  7 purchase totals = $906.35       $993.3236   $993.31   $993.35   $993.20