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 $115.87   13.426%   $131.4267   $131.43   $131.45   $131.40  
  Purchase #2 $124.45   12.704%   $140.2601   $140.26   $140.25   $140.30  
  Purchase #3 $140.67   6.139%   $149.3057   $149.31   $149.30   $149.30  
  Purchase #4 $131.79   11.784%   $147.3201   $147.32   $147.30   $147.30  
  Purchase #5 $124.53   10.673%   $137.8211   $137.82   $137.80   $137.80  
  Purchase #6 $229.59   3.570%   $237.7864   $237.79   $237.80   $237.80  
  Purchase #7 $42.45   8.239%   $45.9475   $45.95   $45.95   $45.90  
  7 purchase totals = $909.35       $989.8676   $989.88   $989.85   $989.80