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 $188.26   12.145%   $211.1242   $211.12   $211.10   $211.10  
  Purchase #2 $151.99   11.361%   $169.2576   $169.26   $169.25   $169.30  
  Purchase #3 $14.41   8.518%   $15.6374   $15.64   $15.65   $15.60  
  Purchase #4 $249.71   7.682%   $268.8927   $268.89   $268.90   $268.90  
  Purchase #5 $71.68   10.257%   $79.0322   $79.03   $79.05   $79.00  
  Purchase #6 $245.71   10.485%   $271.4727   $271.47   $271.45   $271.50  
  Purchase #7 $228.39   8.674%   $248.2005   $248.20   $248.20   $248.20  
  7 purchase totals = $1,150.15       $1,263.6173   $1,263.61   $1,263.60   $1,263.60