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 $239.25   13.402%   $271.3143   $271.31   $271.30   $271.30  
  Purchase #2 $212.43   11.866%   $237.6369   $237.64   $237.65   $237.60  
  Purchase #3 $158.27   7.137%   $169.5657   $169.57   $169.55   $169.60  
  Purchase #4 $108.29   4.517%   $113.1815   $113.18   $113.20   $113.20  
  Purchase #5 $161.80   4.395%   $168.9111   $168.91   $168.90   $168.90  
  Purchase #6 $95.71   10.481%   $105.7414   $105.74   $105.75   $105.70  
  Purchase #7 $168.86   10.512%   $186.6106   $186.61   $186.60   $186.60  
  7 purchase totals = $1,144.61       $1,252.9615   $1,252.96   $1,252.95   $1,252.90