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 $18.46   13.729%   $20.9944   $20.99   $21.00   $21.00  
  Purchase #2 $103.99   13.486%   $118.0141   $118.01   $118.00   $118.00  
  Purchase #3 $222.35   6.278%   $236.3091   $236.31   $236.30   $236.30  
  Purchase #4 $220.48   3.160%   $227.4472   $227.45   $227.45   $227.40  
  Purchase #5 $183.66   10.682%   $203.2786   $203.28   $203.30   $203.30  
  Purchase #6 $30.50   8.354%   $33.048   $33.05   $33.05   $33.00  
  Purchase #7 $46.48   5.661%   $49.1112   $49.11   $49.10   $49.10  
  7 purchase totals = $825.92       $888.2026   $888.20   $888.20   $888.10