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 $239.73   10.696%   $265.3715   $265.37   $265.35   $265.40  
  Purchase #2 $141.88   6.402%   $150.9632   $150.96   $150.95   $151.00  
  Purchase #3 $249.76   10.301%   $275.4878   $275.49   $275.50   $275.50  
  Purchase #4 $35.60   8.232%   $38.5306   $38.53   $38.55   $38.50  
  Purchase #5 $56.78   6.609%   $60.5326   $60.53   $60.55   $60.50  
  Purchase #6 $36.92   12.449%   $41.5162   $41.52   $41.50   $41.50  
  Purchase #7 $187.49   5.601%   $197.9913   $197.99   $198.00   $198.00  
  7 purchase totals = $948.16       $1,030.3932   $1,030.39   $1,030.40   $1,030.40