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 $16.80   9.448%   $18.3873   $18.39   $18.40   $18.40  
  Purchase #2 $139.51   5.400%   $147.0435   $147.04   $147.05   $147.00  
  Purchase #3 $221.91   7.851%   $239.3322   $239.33   $239.35   $239.30  
  Purchase #4 $103.95   10.736%   $115.1101   $115.11   $115.10   $115.10  
  Purchase #5 $29.38   7.741%   $31.6543   $31.65   $31.65   $31.70  
  Purchase #6 $91.36   9.254%   $99.8145   $99.81   $99.80   $99.80  
  Purchase #7 $250.58   11.321%   $278.9482   $278.95   $278.95   $278.90  
  7 purchase totals = $853.49       $930.2901   $930.28   $930.30   $930.20