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 $128.80   5.531%   $135.9239   $135.92   $135.90   $135.90  
  Purchase #2 $225.35   4.385%   $235.2316   $235.23   $235.25   $235.20  
  Purchase #3 $90.90   3.667%   $94.2333   $94.23   $94.25   $94.20  
  Purchase #4 $115.87   11.299%   $128.9622   $128.96   $128.95   $129.00  
  Purchase #5 $155.31   13.227%   $175.8529   $175.85   $175.85   $175.90  
  Purchase #6 $18.54   7.570%   $19.9435   $19.94   $19.95   $19.90  
  Purchase #7 $74.38   6.728%   $79.3843   $79.38   $79.40   $79.40  
  7 purchase totals = $809.15       $869.5317   $869.51   $869.55   $869.50