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 $225.47   12.796%   $254.3211   $254.32   $254.30   $254.30  
  Purchase #2 $64.28   11.633%   $71.7577   $71.76   $71.75   $71.80  
  Purchase #3 $65.27   4.498%   $68.2058   $68.21   $68.20   $68.20  
  Purchase #4 $37.85   4.383%   $39.509   $39.51   $39.50   $39.50  
  Purchase #5 $181.99   13.228%   $206.0636   $206.06   $206.05   $206.10  
  Purchase #6 $49.43   11.258%   $54.9948   $54.99   $55.00   $55.00  
  Purchase #7 $23.86   11.213%   $26.5354   $26.54   $26.55   $26.50  
  7 purchase totals = $648.15       $721.3874   $721.39   $721.35   $721.40