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 $11.84   11.374%   $13.1867   $13.19   $13.20   $13.20  
  Purchase #2 $190.56   6.129%   $202.2394   $202.24   $202.25   $202.20  
  Purchase #3 $57.50   3.113%   $59.29   $59.29   $59.30   $59.30  
  Purchase #4 $149.72   3.445%   $154.8779   $154.88   $154.90   $154.90  
  Purchase #5 $149.55   8.230%   $161.858   $161.86   $161.85   $161.90  
  Purchase #6 $89.29   5.222%   $93.9527   $93.95   $93.95   $94.00  
  Purchase #7 $143.99   6.138%   $152.8281   $152.83   $152.85   $152.80  
  7 purchase totals = $792.45       $838.2328   $838.24   $838.30   $838.30