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 $244.71   11.680%   $273.2921   $273.29   $273.30   $273.30  
  Purchase #2 $129.72   8.194%   $140.3493   $140.35   $140.35   $140.30  
  Purchase #3 $182.62   8.242%   $197.6715   $197.67   $197.65   $197.70  
  Purchase #4 $146.47   6.161%   $155.494   $155.49   $155.50   $155.50  
  Purchase #5 $171.52   11.513%   $191.2671   $191.27   $191.25   $191.30  
  Purchase #6 $233.30   4.115%   $242.9003   $242.90   $242.90   $242.90  
  Purchase #7 $213.60   7.206%   $228.992   $228.99   $229.00   $229.00  
  7 purchase totals = $1,321.94       $1,429.9663   $1,429.96   $1,429.95   $1,430.00