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 $157.82   12.651%   $177.7858   $177.79   $177.80   $177.80  
  Purchase #2 $212.90   10.102%   $234.4072   $234.41   $234.40   $234.40  
  Purchase #3 $204.83   9.757%   $224.8153   $224.82   $224.80   $224.80  
  Purchase #4 $13.29   13.859%   $15.1319   $15.13   $15.15   $15.10  
  Purchase #5 $57.90   10.602%   $64.0386   $64.04   $64.05   $64.00  
  Purchase #6 $226.85   10.532%   $250.7418   $250.74   $250.75   $250.70  
  Purchase #7 $154.98   9.551%   $169.7821   $169.78   $169.80   $169.80  
  7 purchase totals = $1,028.57       $1,136.7027   $1,136.71   $1,136.75   $1,136.60