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 $236.26   12.604%   $266.0382   $266.04   $266.05   $266.00  
  Purchase #2 $150.58   11.321%   $167.6272   $167.63   $167.65   $167.60  
  Purchase #3 $47.54   11.329%   $52.9258   $52.93   $52.95   $52.90  
  Purchase #4 $197.84   11.865%   $221.3137   $221.31   $221.30   $221.30  
  Purchase #5 $30.78   7.478%   $33.0817   $33.08   $33.10   $33.10  
  Purchase #6 $58.85   6.234%   $62.5187   $62.52   $62.50   $62.50  
  Purchase #7 $33.65   3.388%   $34.7901   $34.79   $34.80   $34.80  
  7 purchase totals = $755.50       $838.2954   $838.30   $838.35   $838.20