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 $162.54   12.597%   $183.0152   $183.02   $183.00   $183.00  
  Purchase #2 $107.62   12.591%   $121.1704   $121.17   $121.15   $121.20  
  Purchase #3 $176.26   9.610%   $193.1986   $193.20   $193.20   $193.20  
  Purchase #4 $205.65   11.565%   $229.4334   $229.43   $229.45   $229.40  
  Purchase #5 $48.54   10.366%   $53.5717   $53.57   $53.55   $53.60  
  Purchase #6 $228.56   12.804%   $257.8248   $257.82   $257.80   $257.80  
  Purchase #7 $192.64   5.743%   $203.7033   $203.70   $203.70   $203.70  
  7 purchase totals = $1,121.81       $1,241.9174   $1,241.91   $1,241.85   $1,241.90