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 $243.86   5.408%   $257.0479   $257.05   $257.05   $257.00  
  Purchase #2 $28.55   13.584%   $32.4282   $32.43   $32.45   $32.40  
  Purchase #3 $226.90   13.857%   $258.3415   $258.34   $258.35   $258.30  
  Purchase #4 $24.37   9.408%   $26.6627   $26.66   $26.65   $26.70  
  Purchase #5 $40.57   12.868%   $45.7905   $45.79   $45.80   $45.80  
  Purchase #6 $101.36   12.483%   $114.0128   $114.01   $114.00   $114.00  
  Purchase #7 $51.52   10.709%   $57.0373   $57.04   $57.05   $57.00  
  7 purchase totals = $717.13       $791.3209   $791.32   $791.35   $791.20