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 $27.99   7.397%   $30.0604   $30.06   $30.05   $30.10  
  Purchase #2 $145.49   11.627%   $162.4061   $162.41   $162.40   $162.40  
  Purchase #3 $152.99   8.209%   $165.5489   $165.55   $165.55   $165.50  
  Purchase #4 $13.94   13.733%   $15.8544   $15.85   $15.85   $15.90  
  Purchase #5 $82.54   11.570%   $92.0899   $92.09   $92.10   $92.10  
  Purchase #6 $52.91   8.294%   $57.2984   $57.30   $57.30   $57.30  
  Purchase #7 $39.65   8.645%   $43.0777   $43.08   $43.10   $43.10  
  7 purchase totals = $515.51       $566.3358   $566.34   $566.35   $566.40