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 $178.92   6.766%   $191.0257   $191.03   $191.05   $191.00  
  Purchase #2 $51.98   13.333%   $58.9105   $58.91   $58.90   $58.90  
  Purchase #3 $18.39   10.764%   $20.3695   $20.37   $20.35   $20.40  
  Purchase #4 $240.44   10.738%   $266.2584   $266.26   $266.25   $266.30  
  Purchase #5 $52.74   4.213%   $54.9619   $54.96   $54.95   $55.00  
  Purchase #6 $14.41   4.198%   $15.0149   $15.01   $15.00   $15.00  
  Purchase #7 $106.49   10.702%   $117.8866   $117.89   $117.90   $117.90  
  7 purchase totals = $663.37       $724.4275   $724.43   $724.40   $724.50