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 $34.87   12.505%   $39.2305   $39.23   $39.25   $39.20  
  Purchase #2 $62.84   7.721%   $67.6919   $67.69   $67.70   $67.70  
  Purchase #3 $91.35   6.485%   $97.274   $97.27   $97.25   $97.30  
  Purchase #4 $208.41   11.517%   $232.4126   $232.41   $232.40   $232.40  
  Purchase #5 $158.71   3.432%   $164.1569   $164.16   $164.15   $164.20  
  Purchase #6 $242.28   12.694%   $273.035   $273.04   $273.05   $273.00  
  Purchase #7 $223.96   10.344%   $247.1264   $247.13   $247.15   $247.10  
  7 purchase totals = $1,022.42       $1,120.9273   $1,120.93   $1,120.95   $1,120.90