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 $134.82   12.377%   $151.5067   $151.51   $151.50   $151.50  
  Purchase #2 $36.41   8.351%   $39.4506   $39.45   $39.45   $39.50  
  Purchase #3 $90.51   9.487%   $99.0967   $99.10   $99.10   $99.10  
  Purchase #4 $206.57   13.332%   $234.1099   $234.11   $234.10   $234.10  
  Purchase #5 $164.36   8.260%   $177.9361   $177.94   $177.95   $177.90  
  Purchase #6 $239.82   12.504%   $269.8071   $269.81   $269.80   $269.80  
  Purchase #7 $244.56   3.834%   $253.9364   $253.94   $253.95   $253.90  
  7 purchase totals = $1,117.05       $1,225.8435   $1,225.86   $1,225.85   $1,225.80