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 $19.87   5.639%   $20.9905   $20.99   $21.00   $21.00  
  Purchase #2 $226.68   9.835%   $248.974   $248.97   $248.95   $249.00  
  Purchase #3 $110.99   13.330%   $125.785   $125.79   $125.80   $125.80  
  Purchase #4 $49.63   4.251%   $51.7398   $51.74   $51.75   $51.70  
  Purchase #5 $22.44   13.428%   $25.4532   $25.45   $25.45   $25.50  
  Purchase #6 $228.97   7.153%   $245.3482   $245.35   $245.35   $245.30  
  Purchase #7 $156.76   5.852%   $165.9336   $165.93   $165.95   $165.90  
  7 purchase totals = $815.34       $884.2243   $884.22   $884.25   $884.20