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 $18.81   13.592%   $21.3667   $21.37   $21.35   $21.40  
  Purchase #2 $19.54   9.350%   $21.367   $21.37   $21.35   $21.40  
  Purchase #3 $141.38   5.445%   $149.0781   $149.08   $149.10   $149.10  
  Purchase #4 $44.40   4.519%   $46.4064   $46.41   $46.40   $46.40  
  Purchase #5 $30.88   5.321%   $32.5231   $32.52   $32.50   $32.50  
  Purchase #6 $112.73   6.523%   $120.0834   $120.08   $120.10   $120.10  
  Purchase #7 $17.69   9.692%   $19.4045   $19.40   $19.40   $19.40  
  7 purchase totals = $385.43       $410.2292   $410.23   $410.20   $410.30