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 $152.74   13.441%   $173.2698   $173.27   $173.25   $173.30  
  Purchase #2 $243.62   12.481%   $274.0262   $274.03   $274.05   $274.00  
  Purchase #3 $65.46   11.243%   $72.8197   $72.82   $72.80   $72.80  
  Purchase #4 $72.26   13.219%   $81.812   $81.81   $81.80   $81.80  
  Purchase #5 $22.94   6.346%   $24.3958   $24.40   $24.40   $24.40  
  Purchase #6 $141.59   11.703%   $158.1603   $158.16   $158.15   $158.20  
  Purchase #7 $187.51   5.491%   $197.8062   $197.81   $197.80   $197.80  
  7 purchase totals = $886.12       $982.2900   $982.30   $982.25   $982.30