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 $18.95   10.708%   $20.9792   $20.98   $21.00   $21.00  
  Purchase #2 $20.53   7.449%   $22.0593   $22.06   $22.05   $22.10  
  Purchase #3 $104.64   9.275%   $114.3454   $114.35   $114.35   $114.30  
  Purchase #4 $77.58   7.692%   $83.5475   $83.55   $83.55   $83.50  
  Purchase #5 $137.79   6.797%   $147.1556   $147.16   $147.15   $147.20  
  Purchase #6 $119.98   9.486%   $131.3613   $131.36   $131.35   $131.40  
  Purchase #7 $156.88   7.566%   $168.7495   $168.75   $168.75   $168.70  
  7 purchase totals = $636.35       $688.1978   $688.21   $688.20   $688.20