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 $16.45   9.390%   $17.9947   $17.99   $18.00   $18.00  
  Purchase #2 $11.65   9.550%   $12.7626   $12.76   $12.75   $12.80  
  Purchase #3 $112.57   3.379%   $116.3737   $116.37   $116.35   $116.40  
  Purchase #4 $158.97   6.382%   $169.1155   $169.12   $169.10   $169.10  
  Purchase #5 $49.41   5.510%   $52.1325   $52.13   $52.15   $52.10  
  Purchase #6 $47.96   4.261%   $50.0036   $50.00   $50.00   $50.00  
  Purchase #7 $139.84   13.620%   $158.8862   $158.89   $158.90   $158.90  
  7 purchase totals = $536.85       $577.2688   $577.26   $577.25   $577.30