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 $197.97   11.313%   $220.3663   $220.37   $220.35   $220.40  
  Purchase #2 $157.47   7.134%   $168.7039   $168.70   $168.70   $168.70  
  Purchase #3 $30.66   6.594%   $32.6817   $32.68   $32.70   $32.70  
  Purchase #4 $139.77   8.282%   $151.3458   $151.35   $151.35   $151.30  
  Purchase #5 $207.39   11.446%   $231.1279   $231.13   $231.15   $231.10  
  Purchase #6 $73.64   12.587%   $82.9091   $82.91   $82.90   $82.90  
  Purchase #7 $117.60   9.811%   $129.1377   $129.14   $129.15   $129.10  
  7 purchase totals = $924.50       $1,016.2724   $1,016.28   $1,016.30   $1,016.20