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 $79.91   12.608%   $89.9851   $89.99   $90.00   $90.00  
  Purchase #2 $82.56   7.168%   $88.4779   $88.48   $88.50   $88.50  
  Purchase #3 $169.64   6.221%   $180.1933   $180.19   $180.20   $180.20  
  Purchase #4 $192.34   5.660%   $203.2264   $203.23   $203.25   $203.20  
  Purchase #5 $99.35   7.686%   $106.986   $106.99   $107.00   $107.00  
  Purchase #6 $93.46   8.263%   $101.1826   $101.18   $101.20   $101.20  
  Purchase #7 $17.58   9.606%   $19.2687   $19.27   $19.25   $19.30  
  7 purchase totals = $734.84       $789.3200   $789.33   $789.40   $789.40