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 $90.64   9.640%   $99.3777   $99.38   $99.40   $99.40  
  Purchase #2 $142.84   3.482%   $147.8137   $147.81   $147.80   $147.80  
  Purchase #3 $9.40   12.823%   $10.6054   $10.61   $10.60   $10.60  
  Purchase #4 $146.56   11.736%   $163.7603   $163.76   $163.75   $163.80  
  Purchase #5 $173.30   6.694%   $184.9007   $184.90   $184.90   $184.90  
  Purchase #6 $75.53   10.460%   $83.4304   $83.43   $83.45   $83.40  
  Purchase #7 $139.75   6.482%   $148.8086   $148.81   $148.80   $148.80  
  7 purchase totals = $778.02       $838.6968   $838.70   $838.70   $838.70