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 $131.79   3.209%   $136.0191   $136.02   $136.00   $136.00  
  Purchase #2 $242.36   8.558%   $263.1012   $263.10   $263.10   $263.10  
  Purchase #3 $219.87   11.346%   $244.8165   $244.82   $244.80   $244.80  
  Purchase #4 $24.85   9.766%   $27.2769   $27.28   $27.30   $27.30  
  Purchase #5 $227.71   12.507%   $256.1897   $256.19   $256.20   $256.20  
  Purchase #6 $238.35   4.399%   $248.835   $248.84   $248.85   $248.80  
  Purchase #7 $171.44   4.441%   $179.0537   $179.05   $179.05   $179.10  
  7 purchase totals = $1,256.37       $1,355.2921   $1,355.30   $1,355.30   $1,355.30