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 $19.54   6.587%   $20.8271   $20.83   $20.85   $20.80  
  Purchase #2 $24.57   11.655%   $27.4336   $27.43   $27.45   $27.40  
  Purchase #3 $238.25   6.193%   $253.0048   $253.00   $253.00   $253.00  
  Purchase #4 $147.25   10.175%   $162.2327   $162.23   $162.25   $162.20  
  Purchase #5 $124.58   12.786%   $140.5088   $140.51   $140.50   $140.50  
  Purchase #6 $233.40   10.574%   $258.0797   $258.08   $258.10   $258.10  
  Purchase #7 $123.60   3.731%   $128.2115   $128.21   $128.20   $128.20  
  7 purchase totals = $911.19       $990.2982   $990.29   $990.35   $990.20