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 $21.29   3.273%   $21.9868   $21.99   $22.00   $22.00  
  Purchase #2 $83.53   3.733%   $86.6482   $86.65   $86.65   $86.60  
  Purchase #3 $204.80   7.824%   $220.8236   $220.82   $220.80   $220.80  
  Purchase #4 $99.25   7.115%   $106.3116   $106.31   $106.30   $106.30  
  Purchase #5 $60.79   4.504%   $63.528   $63.53   $63.55   $63.50  
  Purchase #6 $114.79   10.537%   $126.8854   $126.89   $126.90   $126.90  
  Purchase #7 $20.27   7.855%   $21.8622   $21.86   $21.85   $21.90  
  7 purchase totals = $604.72       $648.0458   $648.05   $648.05   $648.00