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 $250.32   13.336%   $283.7027   $283.70   $283.70   $283.70  
  Purchase #2 $67.58   9.662%   $74.1096   $74.11   $74.10   $74.10  
  Purchase #3 $105.39   7.762%   $113.5704   $113.57   $113.55   $113.60  
  Purchase #4 $35.48   12.308%   $39.8469   $39.85   $39.85   $39.80  
  Purchase #5 $32.66   5.595%   $34.4873   $34.49   $34.50   $34.50  
  Purchase #6 $220.30   3.819%   $228.7133   $228.71   $228.70   $228.70  
  Purchase #7 $145.30   10.473%   $160.5173   $160.52   $160.50   $160.50  
  7 purchase totals = $857.03       $934.9475   $934.95   $934.90   $934.90