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 $239.67   7.402%   $257.4104   $257.41   $257.40   $257.40  
  Purchase #2 $160.93   8.512%   $174.6284   $174.63   $174.65   $174.60  
  Purchase #3 $149.97   10.400%   $165.5669   $165.57   $165.55   $165.60  
  Purchase #4 $216.98   9.173%   $236.8836   $236.88   $236.90   $236.90  
  Purchase #5 $27.94   13.283%   $31.6513   $31.65   $31.65   $31.70  
  Purchase #6 $166.93   12.197%   $187.2905   $187.29   $187.30   $187.30  
  Purchase #7 $220.63   13.668%   $250.7857   $250.79   $250.80   $250.80  
  7 purchase totals = $1,183.05       $1,304.2168   $1,304.22   $1,304.25   $1,304.30