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 $12.50   3.861%   $12.9826   $12.98   $13.00   $13.00  
  Purchase #2 $148.94   4.190%   $155.1806   $155.18   $155.20   $155.20  
  Purchase #3 $23.68   4.169%   $24.6672   $24.67   $24.65   $24.70  
  Purchase #4 $185.61   6.378%   $197.4482   $197.45   $197.45   $197.40  
  Purchase #5 $155.72   11.572%   $173.7399   $173.74   $173.75   $173.70  
  Purchase #6 $26.51   5.137%   $27.8718   $27.87   $27.85   $27.90  
  Purchase #7 $45.73   12.346%   $51.3758   $51.38   $51.40   $51.40  
  7 purchase totals = $598.69       $643.2661   $643.27   $643.30   $643.30