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 $248.44   8.286%   $269.0257   $269.03   $269.05   $269.00  
  Purchase #2 $169.30   9.374%   $185.1702   $185.17   $185.15   $185.20  
  Purchase #3 $33.70   9.606%   $36.9372   $36.94   $36.95   $36.90  
  Purchase #4 $36.65   8.466%   $39.7528   $39.75   $39.75   $39.80  
  Purchase #5 $27.67   12.732%   $31.1929   $31.19   $31.20   $31.20  
  Purchase #6 $6.64   5.631%   $7.0139   $7.01   $7.00   $7.00  
  Purchase #7 $85.40   4.117%   $88.9159   $88.92   $88.90   $88.90  
  7 purchase totals = $607.80       $658.0086   $658.01   $658.00   $658.00