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 $138.40   8.234%   $149.7959   $149.80   $149.80   $149.80  
  Purchase #2 $208.55   8.388%   $226.0432   $226.04   $226.05   $226.00  
  Purchase #3 $212.40   8.508%   $230.471   $230.47   $230.45   $230.50  
  Purchase #4 $48.68   5.176%   $51.1997   $51.20   $51.20   $51.20  
  Purchase #5 $236.37   9.779%   $259.4846   $259.48   $259.50   $259.50  
  Purchase #6 $201.85   7.560%   $217.1099   $217.11   $217.10   $217.10  
  Purchase #7 $217.86   6.823%   $232.7246   $232.72   $232.70   $232.70  
  7 purchase totals = $1,264.11       $1,366.8289   $1,366.82   $1,366.80   $1,366.80