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 $235.42   12.151%   $264.0259   $264.03   $264.05   $264.00  
  Purchase #2 $88.99   6.378%   $94.6658   $94.67   $94.65   $94.70  
  Purchase #3 $193.33   5.842%   $204.6243   $204.62   $204.60   $204.60  
  Purchase #4 $118.25   7.451%   $127.0608   $127.06   $127.05   $127.10  
  Purchase #5 $199.33   3.284%   $205.876   $205.88   $205.90   $205.90  
  Purchase #6 $211.86   8.214%   $229.2622   $229.26   $229.25   $229.30  
  Purchase #7 $196.37   8.571%   $213.2009   $213.20   $213.20   $213.20  
  7 purchase totals = $1,243.55       $1,338.7159   $1,338.72   $1,338.70   $1,338.80