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 $15.39   9.444%   $16.8434   $16.84   $16.85   $16.80  
  Purchase #2 $14.95   13.449%   $16.9606   $16.96   $16.95   $17.00  
  Purchase #3 $90.36   5.747%   $95.553   $95.55   $95.55   $95.60  
  Purchase #4 $217.65   4.239%   $226.8762   $226.88   $226.90   $226.90  
  Purchase #5 $14.59   10.667%   $16.1463   $16.15   $16.15   $16.10  
  Purchase #6 $27.77   7.680%   $29.9027   $29.90   $29.90   $29.90  
  Purchase #7 $225.89   3.373%   $233.5093   $233.51   $233.50   $233.50  
  7 purchase totals = $606.60       $635.7915   $635.79   $635.80   $635.80