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 $17.90   3.369%   $18.5031   $18.50   $18.50   $18.50  
  Purchase #2 $227.75   13.510%   $258.519   $258.52   $258.50   $258.50  
  Purchase #3 $174.28   7.250%   $186.9153   $186.92   $186.90   $186.90  
  Purchase #4 $191.94   13.640%   $218.1206   $218.12   $218.10   $218.10  
  Purchase #5 $192.99   11.126%   $214.4621   $214.46   $214.45   $214.50  
  Purchase #6 $153.64   9.293%   $167.9178   $167.92   $167.90   $167.90  
  Purchase #7 $60.59   7.564%   $65.173   $65.17   $65.15   $65.20  
  7 purchase totals = $1,019.09       $1,129.6109   $1,129.61   $1,129.50   $1,129.60