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 $18.89   6.419%   $20.1025   $20.10   $20.10   $20.10  
  Purchase #2 $178.51   8.632%   $193.919   $193.92   $193.90   $193.90  
  Purchase #3 $221.97   8.272%   $240.3314   $240.33   $240.35   $240.30  
  Purchase #4 $23.35   12.817%   $26.3428   $26.34   $26.35   $26.30  
  Purchase #5 $16.81   4.705%   $17.6009   $17.60   $17.60   $17.60  
  Purchase #6 $46.48   10.254%   $51.2461   $51.25   $51.25   $51.20  
  Purchase #7 $63.64   12.135%   $71.3627   $71.36   $71.35   $71.40  
  7 purchase totals = $569.65       $620.9054   $620.90   $620.90   $620.80