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 $27.32   13.448%   $30.994   $30.99   $31.00   $31.00  
  Purchase #2 $30.40   12.223%   $34.1158   $34.12   $34.10   $34.10  
  Purchase #3 $169.30   13.457%   $192.0827   $192.08   $192.10   $192.10  
  Purchase #4 $93.84   6.616%   $100.0485   $100.05   $100.05   $100.00  
  Purchase #5 $97.96   11.235%   $108.9658   $108.97   $108.95   $109.00  
  Purchase #6 $248.48   5.564%   $262.3054   $262.31   $262.30   $262.30  
  Purchase #7 $46.60   6.114%   $49.4491   $49.45   $49.45   $49.40  
  7 purchase totals = $713.90       $777.9613   $777.97   $777.95   $777.90