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 $127.65   6.865%   $136.4132   $136.41   $136.40   $136.40  
  Purchase #2 $111.66   7.499%   $120.0334   $120.03   $120.05   $120.00  
  Purchase #3 $31.39   8.705%   $34.1225   $34.12   $34.10   $34.10  
  Purchase #4 $219.29   5.289%   $230.8882   $230.89   $230.90   $230.90  
  Purchase #5 $209.63   12.727%   $236.3096   $236.31   $236.30   $236.30  
  Purchase #6 $119.73   7.398%   $128.5876   $128.59   $128.60   $128.60  
  Purchase #7 $95.57   4.698%   $100.0599   $100.06   $100.05   $100.10  
  7 purchase totals = $914.92       $986.4144   $986.41   $986.40   $986.40