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 $114.59   12.596%   $129.0238   $129.02   $129.00   $129.00  
  Purchase #2 $23.41   5.199%   $24.6271   $24.63   $24.65   $24.60  
  Purchase #3 $180.48   11.364%   $200.9897   $200.99   $201.00   $201.00  
  Purchase #4 $204.35   12.433%   $229.7568   $229.76   $229.75   $229.80  
  Purchase #5 $109.65   10.665%   $121.3442   $121.34   $121.35   $121.30  
  Purchase #6 $239.50   5.572%   $252.8449   $252.84   $252.85   $252.80  
  Purchase #7 $62.58   12.147%   $70.1816   $70.18   $70.20   $70.20  
  7 purchase totals = $934.56       $1,028.7681   $1,028.76   $1,028.80   $1,028.70