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 $29.48   4.556%   $30.8231   $30.82   $30.80   $30.80  
  Purchase #2 $17.44   5.697%   $18.4336   $18.43   $18.45   $18.40  
  Purchase #3 $218.51   3.589%   $226.3523   $226.35   $226.35   $226.40  
  Purchase #4 $214.83   4.802%   $225.1461   $225.15   $225.15   $225.10  
  Purchase #5 $26.46   11.143%   $29.4084   $29.41   $29.40   $29.40  
  Purchase #6 $105.62   5.339%   $111.2591   $111.26   $111.25   $111.30  
  Purchase #7 $15.52   6.206%   $16.4832   $16.48   $16.50   $16.50  
  7 purchase totals = $627.86       $657.9058   $657.90   $657.90   $657.90