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 $56.78   5.634%   $59.979   $59.98   $60.00   $60.00  
  Purchase #2 $17.81   9.644%   $19.5276   $19.53   $19.55   $19.50  
  Purchase #3 $168.30   13.751%   $191.4429   $191.44   $191.45   $191.40  
  Purchase #4 $65.56   12.533%   $73.7766   $73.78   $73.80   $73.80  
  Purchase #5 $187.99   13.769%   $213.8743   $213.87   $213.85   $213.90  
  Purchase #6 $112.76   3.107%   $116.2635   $116.26   $116.25   $116.30  
  Purchase #7 $179.74   12.335%   $201.9109   $201.91   $201.90   $201.90  
  7 purchase totals = $788.94       $876.7748   $876.77   $876.80   $876.80