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 $236.29   6.383%   $251.3724   $251.37   $251.35   $251.40  
  Purchase #2 $168.57   12.778%   $190.1099   $190.11   $190.10   $190.10  
  Purchase #3 $93.48   5.424%   $98.5504   $98.55   $98.55   $98.60  
  Purchase #4 $179.70   12.202%   $201.627   $201.63   $201.65   $201.60  
  Purchase #5 $24.94   5.303%   $26.2626   $26.26   $26.25   $26.30  
  Purchase #6 $126.81   11.639%   $141.5694   $141.57   $141.55   $141.60  
  Purchase #7 $165.31   11.201%   $183.8264   $183.83   $183.85   $183.80  
  7 purchase totals = $995.10       $1,093.3181   $1,093.32   $1,093.30   $1,093.40