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 $205.79   11.755%   $229.9806   $229.98   $230.00   $230.00  
  Purchase #2 $95.53   12.690%   $107.6528   $107.65   $107.65   $107.70  
  Purchase #3 $231.53   7.413%   $248.6933   $248.69   $248.70   $248.70  
  Purchase #4 $48.58   4.601%   $50.8152   $50.82   $50.80   $50.80  
  Purchase #5 $219.58   12.749%   $247.5743   $247.57   $247.55   $247.60  
  Purchase #6 $165.46   5.105%   $173.9067   $173.91   $173.90   $173.90  
  Purchase #7 $220.82   8.510%   $239.6118   $239.61   $239.60   $239.60  
  7 purchase totals = $1,187.29       $1,298.2347   $1,298.23   $1,298.20   $1,298.30