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 $236.35   7.654%   $254.4402   $254.44   $254.45   $254.40  
  Purchase #2 $121.84   8.465%   $132.1538   $132.15   $132.15   $132.20  
  Purchase #3 $80.90   4.499%   $84.5397   $84.54   $84.55   $84.50  
  Purchase #4 $110.99   5.427%   $117.0134   $117.01   $117.00   $117.00  
  Purchase #5 $58.98   11.526%   $65.778   $65.78   $65.80   $65.80  
  Purchase #6 $131.33   4.372%   $137.0717   $137.07   $137.05   $137.10  
  Purchase #7 $35.78   8.694%   $38.8907   $38.89   $38.90   $38.90  
  7 purchase totals = $776.17       $829.8875   $829.88   $829.90   $829.90