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 $243.25   12.659%   $274.043   $274.04   $274.05   $274.00  
  Purchase #2 $200.37   13.729%   $227.8788   $227.88   $227.90   $227.90  
  Purchase #3 $219.81   13.174%   $248.7678   $248.77   $248.75   $248.80  
  Purchase #4 $242.58   10.356%   $267.7016   $267.70   $267.70   $267.70  
  Purchase #5 $103.31   11.793%   $115.4933   $115.49   $115.50   $115.50  
  Purchase #6 $234.37   13.355%   $265.6701   $265.67   $265.65   $265.70  
  Purchase #7 $223.34   12.361%   $250.9471   $250.95   $250.95   $250.90  
  7 purchase totals = $1,467.03       $1,650.5017   $1,650.50   $1,650.50   $1,650.50