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 $239.41   7.356%   $257.021   $257.02   $257.00   $257.00  
  Purchase #2 $135.40   13.620%   $153.8415   $153.84   $153.85   $153.80  
  Purchase #3 $163.81   9.112%   $178.7364   $178.74   $178.75   $178.70  
  Purchase #4 $112.78   6.297%   $119.8818   $119.88   $119.90   $119.90  
  Purchase #5 $6.90   10.359%   $7.6148   $7.61   $7.60   $7.60  
  Purchase #6 $227.37   9.382%   $248.7019   $248.70   $248.70   $248.70  
  Purchase #7 $60.61   6.142%   $64.3327   $64.33   $64.35   $64.30  
  7 purchase totals = $946.28       $1,030.1301   $1,030.12   $1,030.15   $1,030.00