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 $12.40   5.782%   $13.117   $13.12   $13.10   $13.10  
  Purchase #2 $239.67   13.586%   $272.2316   $272.23   $272.25   $272.20  
  Purchase #3 $114.88   12.857%   $129.6501   $129.65   $129.65   $129.70  
  Purchase #4 $226.52   4.240%   $236.1244   $236.12   $236.10   $236.10  
  Purchase #5 $180.45   13.644%   $205.0706   $205.07   $205.05   $205.10  
  Purchase #6 $47.69   10.592%   $52.7413   $52.74   $52.75   $52.70  
  Purchase #7 $108.55   4.511%   $113.4467   $113.45   $113.45   $113.40  
  7 purchase totals = $930.16       $1,022.3817   $1,022.38   $1,022.35   $1,022.30