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 $48.40   6.225%   $51.4129   $51.41   $51.40   $51.40  
  Purchase #2 $239.88   13.616%   $272.5421   $272.54   $272.55   $272.50  
  Purchase #3 $138.55   6.290%   $147.2648   $147.26   $147.25   $147.30  
  Purchase #4 $7.99   9.113%   $8.7181   $8.72   $8.70   $8.70  
  Purchase #5 $193.84   3.843%   $201.2893   $201.29   $201.30   $201.30  
  Purchase #6 $51.77   13.536%   $58.7776   $58.78   $58.80   $58.80  
  Purchase #7 $220.47   13.508%   $250.2511   $250.25   $250.25   $250.30  
  7 purchase totals = $900.90       $990.2559   $990.25   $990.25   $990.30