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 $240.89   12.359%   $270.6616   $270.66   $270.65   $270.70  
  Purchase #2 $115.53   9.694%   $126.7295   $126.73   $126.75   $126.70  
  Purchase #3 $164.72   4.567%   $172.2428   $172.24   $172.25   $172.20  
  Purchase #4 $82.46   3.505%   $85.3502   $85.35   $85.35   $85.40  
  Purchase #5 $62.75   9.655%   $68.8085   $68.81   $68.80   $68.80  
  Purchase #6 $144.44   12.560%   $162.5817   $162.58   $162.60   $162.60  
  Purchase #7 $158.68   5.250%   $167.0107   $167.01   $167.00   $167.00  
  7 purchase totals = $969.47       $1,053.3850   $1,053.38   $1,053.40   $1,053.40