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 $113.69   8.491%   $123.3434   $123.34   $123.35   $123.30  
  Purchase #2 $233.94   13.258%   $264.9558   $264.96   $264.95   $265.00  
  Purchase #3 $237.90   8.406%   $257.8979   $257.90   $257.90   $257.90  
  Purchase #4 $18.75   7.297%   $20.1182   $20.12   $20.10   $20.10  
  Purchase #5 $101.56   10.442%   $112.1649   $112.16   $112.15   $112.20  
  Purchase #6 $226.29   3.625%   $234.493   $234.49   $234.50   $234.50  
  Purchase #7 $238.40   9.831%   $261.8371   $261.84   $261.85   $261.80  
  7 purchase totals = $1,170.53       $1,274.8103   $1,274.81   $1,274.80   $1,274.80