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 $199.98   10.204%   $220.386   $220.39   $220.40   $220.40  
  Purchase #2 $164.40   7.530%   $176.7793   $176.78   $176.80   $176.80  
  Purchase #3 $120.70   8.715%   $131.219   $131.22   $131.20   $131.20  
  Purchase #4 $214.50   10.354%   $236.7093   $236.71   $236.70   $236.70  
  Purchase #5 $110.41   4.183%   $115.0285   $115.03   $115.05   $115.00  
  Purchase #6 $139.61   9.492%   $152.8618   $152.86   $152.85   $152.90  
  Purchase #7 $33.27   13.745%   $37.843   $37.84   $37.85   $37.80  
  7 purchase totals = $982.87       $1,070.8269   $1,070.83   $1,070.85   $1,070.80