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 $134.31   3.313%   $138.7597   $138.76   $138.75   $138.80  
  Purchase #2 $68.44   3.516%   $70.8464   $70.85   $70.85   $70.80  
  Purchase #3 $69.73   8.591%   $75.7205   $75.72   $75.70   $75.70  
  Purchase #4 $75.36   12.540%   $84.8101   $84.81   $84.80   $84.80  
  Purchase #5 $172.42   9.733%   $189.2016   $189.20   $189.20   $189.20  
  Purchase #6 $39.79   4.124%   $41.4309   $41.43   $41.45   $41.40  
  Purchase #7 $40.73   7.549%   $43.8047   $43.80   $43.80   $43.80  
  7 purchase totals = $600.78       $644.5739   $644.57   $644.55   $644.50