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 $80.50   9.775%   $88.3689   $88.37   $88.35   $88.40  
  Purchase #2 $37.83   7.699%   $40.7425   $40.74   $40.75   $40.70  
  Purchase #3 $170.89   5.365%   $180.0582   $180.06   $180.05   $180.10  
  Purchase #4 $250.94   6.283%   $266.7066   $266.71   $266.70   $266.70  
  Purchase #5 $20.39   6.765%   $21.7694   $21.77   $21.75   $21.80  
  Purchase #6 $138.56   12.259%   $155.5461   $155.55   $155.55   $155.50  
  Purchase #7 $119.99   11.818%   $134.1704   $134.17   $134.15   $134.20  
  7 purchase totals = $819.10       $887.3621   $887.37   $887.30   $887.40