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 $26.93   5.605%   $28.4394   $28.44   $28.45   $28.40  
  Purchase #2 $205.60   7.764%   $221.5628   $221.56   $221.55   $221.60  
  Purchase #3 $224.78   8.273%   $243.376   $243.38   $243.40   $243.40  
  Purchase #4 $25.26   12.123%   $28.3223   $28.32   $28.30   $28.30  
  Purchase #5 $149.54   6.211%   $158.8279   $158.83   $158.85   $158.80  
  Purchase #6 $78.30   4.174%   $81.5682   $81.57   $81.55   $81.60  
  Purchase #7 $69.87   11.546%   $77.9372   $77.94   $77.95   $77.90  
  7 purchase totals = $780.28       $840.0338   $840.04   $840.05   $840.00