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 $6.45   5.292%   $6.7913   $6.79   $6.80   $6.80  
  Purchase #2 $206.85   6.219%   $219.714   $219.71   $219.70   $219.70  
  Purchase #3 $178.79   4.821%   $187.4095   $187.41   $187.40   $187.40  
  Purchase #4 $31.75   7.789%   $34.223   $34.22   $34.20   $34.20  
  Purchase #5 $177.65   5.243%   $186.9642   $186.96   $186.95   $187.00  
  Purchase #6 $120.85   12.431%   $135.8729   $135.87   $135.85   $135.90  
  Purchase #7 $136.80   10.682%   $151.413   $151.41   $151.40   $151.40  
  7 purchase totals = $859.14       $922.3879   $922.37   $922.30   $922.40