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 $166.65   13.522%   $189.1844   $189.18   $189.20   $189.20  
  Purchase #2 $143.99   6.844%   $153.8447   $153.84   $153.85   $153.80  
  Purchase #3 $96.45   9.778%   $105.8809   $105.88   $105.90   $105.90  
  Purchase #4 $103.87   13.247%   $117.6297   $117.63   $117.65   $117.60  
  Purchase #5 $184.56   13.247%   $209.0087   $209.01   $209.00   $209.00  
  Purchase #6 $239.30   6.202%   $254.1414   $254.14   $254.15   $254.10  
  Purchase #7 $227.94   5.295%   $240.0094   $240.01   $240.00   $240.00  
  7 purchase totals = $1,162.76       $1,269.6992   $1,269.69   $1,269.75   $1,269.60