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 $52.95   8.166%   $57.2739   $57.27   $57.25   $57.30  
  Purchase #2 $234.77   13.271%   $265.9263   $265.93   $265.95   $265.90  
  Purchase #3 $159.36   6.595%   $169.8698   $169.87   $169.85   $169.90  
  Purchase #4 $201.95   5.398%   $212.8513   $212.85   $212.85   $212.90  
  Purchase #5 $106.55   11.679%   $118.994   $118.99   $119.00   $119.00  
  Purchase #6 $38.40   3.803%   $39.8604   $39.86   $39.85   $39.90  
  Purchase #7 $139.51   11.486%   $155.5341   $155.53   $155.55   $155.50  
  7 purchase totals = $933.49       $1,020.3098   $1,020.30   $1,020.30   $1,020.40