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 $9.99   3.839%   $10.3735   $10.37   $10.35   $10.40  
  Purchase #2 $35.25   10.530%   $38.9618   $38.96   $38.95   $39.00  
  Purchase #3 $12.43   8.705%   $13.512   $13.51   $13.50   $13.50  
  Purchase #4 $82.26   12.676%   $92.6873   $92.69   $92.70   $92.70  
  Purchase #5 $220.64   7.606%   $237.4219   $237.42   $237.40   $237.40  
  Purchase #6 $238.32   12.146%   $267.2663   $267.27   $267.25   $267.30  
  Purchase #7 $187.92   12.336%   $211.1018   $211.10   $211.10   $211.10  
  7 purchase totals = $786.81       $871.3246   $871.32   $871.25   $871.40