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 $30.97   8.144%   $33.4922   $33.49   $33.50   $33.50  
  Purchase #2 $96.65   3.438%   $99.9728   $99.97   $99.95   $100.00  
  Purchase #3 $95.25   13.166%   $107.7906   $107.79   $107.80   $107.80  
  Purchase #4 $29.57   11.625%   $33.0075   $33.01   $33.00   $33.00  
  Purchase #5 $95.52   12.523%   $107.482   $107.48   $107.50   $107.50  
  Purchase #6 $29.38   4.521%   $30.7083   $30.71   $30.70   $30.70  
  Purchase #7 $139.89   13.766%   $159.1473   $159.15   $159.15   $159.10  
  7 purchase totals = $517.23       $571.6007   $571.60   $571.60   $571.60