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 $133.35   7.786%   $143.7326   $143.73   $143.75   $143.70  
  Purchase #2 $42.45   12.373%   $47.7023   $47.70   $47.70   $47.70  
  Purchase #3 $20.91   6.121%   $22.1899   $22.19   $22.20   $22.20  
  Purchase #4 $201.99   13.379%   $229.0142   $229.01   $229.00   $229.00  
  Purchase #5 $21.90   9.763%   $24.0381   $24.04   $24.05   $24.00  
  Purchase #6 $37.81   8.775%   $41.1278   $41.13   $41.15   $41.10  
  Purchase #7 $78.31   4.674%   $81.9702   $81.97   $81.95   $82.00  
  7 purchase totals = $536.72       $589.7751   $589.77   $589.80   $589.70