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 $7.70   13.226%   $8.7184   $8.72   $8.70   $8.70  
  Purchase #2 $199.25   8.180%   $215.5487   $215.55   $215.55   $215.50  
  Purchase #3 $22.39   12.819%   $25.2602   $25.26   $25.25   $25.30  
  Purchase #4 $249.70   12.149%   $280.0361   $280.04   $280.05   $280.00  
  Purchase #5 $68.86   8.279%   $74.5609   $74.56   $74.55   $74.60  
  Purchase #6 $70.95   12.839%   $80.0593   $80.06   $80.05   $80.10  
  Purchase #7 $101.48   3.360%   $104.8897   $104.89   $104.90   $104.90  
  7 purchase totals = $720.33       $789.0733   $789.08   $789.05   $789.10