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 $172.89   7.228%   $185.3865   $185.39   $185.40   $185.40  
  Purchase #2 $22.42   9.552%   $24.5616   $24.56   $24.55   $24.60  
  Purchase #3 $137.84   12.622%   $155.2382   $155.24   $155.25   $155.20  
  Purchase #4 $115.79   11.367%   $128.9518   $128.95   $128.95   $129.00  
  Purchase #5 $117.62   7.715%   $126.6944   $126.69   $126.70   $126.70  
  Purchase #6 $145.37   9.745%   $159.5363   $159.54   $159.55   $159.50  
  Purchase #7 $99.75   9.246%   $108.9729   $108.97   $108.95   $109.00  
  7 purchase totals = $811.68       $889.3417   $889.34   $889.35   $889.40