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 $232.76   6.860%   $248.7273   $248.73   $248.75   $248.70  
  Purchase #2 $28.81   3.224%   $29.7388   $29.74   $29.75   $29.70  
  Purchase #3 $94.27   3.755%   $97.8098   $97.81   $97.80   $97.80  
  Purchase #4 $195.96   13.396%   $222.2108   $222.21   $222.20   $222.20  
  Purchase #5 $77.68   9.184%   $84.8141   $84.81   $84.80   $84.80  
  Purchase #6 $118.36   11.811%   $132.3395   $132.34   $132.35   $132.30  
  Purchase #7 $102.29   5.313%   $107.7247   $107.72   $107.70   $107.70  
  7 purchase totals = $850.13       $923.3650   $923.36   $923.35   $923.20