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 $91.51   8.382%   $99.1804   $99.18   $99.20   $99.20  
  Purchase #2 $228.67   9.289%   $249.9112   $249.91   $249.90   $249.90  
  Purchase #3 $194.33   11.311%   $216.3107   $216.31   $216.30   $216.30  
  Purchase #4 $242.98   9.216%   $265.373   $265.37   $265.35   $265.40  
  Purchase #5 $234.79   9.271%   $256.5574   $256.56   $256.55   $256.60  
  Purchase #6 $207.93   13.556%   $236.117   $236.12   $236.10   $236.10  
  Purchase #7 $89.60   6.760%   $95.657   $95.66   $95.65   $95.70  
  7 purchase totals = $1,289.81       $1,419.1067   $1,419.11   $1,419.05   $1,419.20