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 $171.25   8.146%   $185.2   $185.20   $185.20   $185.20  
  Purchase #2 $16.36   8.148%   $17.693   $17.69   $17.70   $17.70  
  Purchase #3 $157.76   6.388%   $167.8377   $167.84   $167.85   $167.80  
  Purchase #4 $88.29   8.440%   $95.7417   $95.74   $95.75   $95.70  
  Purchase #5 $221.91   7.709%   $239.017   $239.02   $239.00   $239.00  
  Purchase #6 $92.26   9.158%   $100.7092   $100.71   $100.70   $100.70  
  Purchase #7 $191.27   12.720%   $215.5995   $215.60   $215.60   $215.60  
  7 purchase totals = $939.10       $1,021.7981   $1,021.80   $1,021.80   $1,021.70