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 $20.79   8.299%   $22.5154   $22.52   $22.50   $22.50  
  Purchase #2 $240.79   4.283%   $251.103   $251.10   $251.10   $251.10  
  Purchase #3 $142.74   5.486%   $150.5707   $150.57   $150.55   $150.60  
  Purchase #4 $87.68   3.782%   $90.9961   $91.00   $91.00   $91.00  
  Purchase #5 $144.73   9.806%   $158.9222   $158.92   $158.90   $158.90  
  Purchase #6 $153.77   10.730%   $170.2695   $170.27   $170.25   $170.30  
  Purchase #7 $35.34   10.552%   $39.0691   $39.07   $39.05   $39.10  
  7 purchase totals = $825.84       $883.4460   $883.45   $883.35   $883.50