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 $17.68   4.495%   $18.4747   $18.47   $18.45   $18.50  
  Purchase #2 $89.68   8.416%   $97.2275   $97.23   $97.25   $97.20  
  Purchase #3 $237.28   12.152%   $266.1143   $266.11   $266.10   $266.10  
  Purchase #4 $112.62   11.565%   $125.6445   $125.64   $125.65   $125.60  
  Purchase #5 $34.30   5.113%   $36.0538   $36.05   $36.05   $36.10  
  Purchase #6 $89.51   8.626%   $97.2311   $97.23   $97.25   $97.20  
  Purchase #7 $132.68   13.140%   $150.1142   $150.11   $150.10   $150.10  
  7 purchase totals = $713.75       $790.8601   $790.84   $790.85   $790.80