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 $15.51   4.237%   $16.1672   $16.17   $16.15   $16.20  
  Purchase #2 $15.79   6.700%   $16.8479   $16.85   $16.85   $16.80  
  Purchase #3 $28.47   5.582%   $30.0592   $30.06   $30.05   $30.10  
  Purchase #4 $221.30   12.735%   $249.4826   $249.48   $249.50   $249.50  
  Purchase #5 $83.28   9.250%   $90.9834   $90.98   $91.00   $91.00  
  Purchase #6 $188.80   10.353%   $208.3465   $208.35   $208.35   $208.30  
  Purchase #7 $159.78   6.495%   $170.1577   $170.16   $170.15   $170.20  
  7 purchase totals = $712.93       $782.0445   $782.05   $782.05   $782.10