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 $18.35   13.830%   $20.8878   $20.89   $20.90   $20.90  
  Purchase #2 $199.30   11.304%   $221.8289   $221.83   $221.85   $221.80  
  Purchase #3 $143.96   3.103%   $148.4271   $148.43   $148.45   $148.40  
  Purchase #4 $160.60   13.143%   $181.7077   $181.71   $181.70   $181.70  
  Purchase #5 $241.92   12.338%   $271.7681   $271.77   $271.75   $271.80  
  Purchase #6 $7.84   11.678%   $8.7556   $8.76   $8.75   $8.80  
  Purchase #7 $171.33   7.522%   $184.2174   $184.22   $184.20   $184.20  
  7 purchase totals = $943.30       $1,037.5926   $1,037.61   $1,037.60   $1,037.60