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 $167.89   13.362%   $190.3235   $190.32   $190.30   $190.30  
  Purchase #2 $198.76   9.499%   $217.6402   $217.64   $217.65   $217.60  
  Purchase #3 $119.68   13.257%   $135.546   $135.55   $135.55   $135.50  
  Purchase #4 $51.67   9.370%   $56.5115   $56.51   $56.50   $56.50  
  Purchase #5 $49.99   4.823%   $52.401   $52.40   $52.40   $52.40  
  Purchase #6 $228.64   6.850%   $244.3018   $244.30   $244.30   $244.30  
  Purchase #7 $67.58   10.129%   $74.4252   $74.43   $74.45   $74.40  
  7 purchase totals = $884.21       $971.1492   $971.15   $971.15   $971.00