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 $66.63   10.152%   $73.3943   $73.39   $73.40   $73.40  
  Purchase #2 $242.31   6.781%   $258.741   $258.74   $258.75   $258.70  
  Purchase #3 $63.91   3.137%   $65.9149   $65.91   $65.90   $65.90  
  Purchase #4 $164.65   9.171%   $179.7501   $179.75   $179.75   $179.80  
  Purchase #5 $215.75   3.505%   $223.312   $223.31   $223.30   $223.30  
  Purchase #6 $25.29   7.213%   $27.1142   $27.11   $27.10   $27.10  
  Purchase #7 $147.98   5.279%   $155.7919   $155.79   $155.80   $155.80  
  7 purchase totals = $926.52       $984.0184   $984.00   $984.00   $984.00