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 $49.53   4.171%   $51.5959   $51.60   $51.60   $51.60  
  Purchase #2 $237.48   13.502%   $269.5445   $269.54   $269.55   $269.50  
  Purchase #3 $124.28   12.640%   $139.989   $139.99   $140.00   $140.00  
  Purchase #4 $149.87   9.634%   $164.3085   $164.31   $164.30   $164.30  
  Purchase #5 $24.33   3.657%   $25.2197   $25.22   $25.20   $25.20  
  Purchase #6 $199.53   13.248%   $225.9637   $225.96   $225.95   $226.00  
  Purchase #7 $145.35   5.502%   $153.3472   $153.35   $153.35   $153.30  
  7 purchase totals = $930.37       $1,029.9685   $1,029.97   $1,029.95   $1,029.90