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 $131.59   12.746%   $148.3625   $148.36   $148.35   $148.40  
  Purchase #2 $142.80   11.620%   $159.3934   $159.39   $159.40   $159.40  
  Purchase #3 $172.26   10.257%   $189.9287   $189.93   $189.95   $189.90  
  Purchase #4 $127.97   7.822%   $137.9798   $137.98   $138.00   $138.00  
  Purchase #5 $74.85   6.729%   $79.8867   $79.89   $79.90   $79.90  
  Purchase #6 $60.65   11.504%   $67.6272   $67.63   $67.65   $67.60  
  Purchase #7 $119.88   8.322%   $129.8564   $129.86   $129.85   $129.90  
  7 purchase totals = $830.00       $913.0347   $913.04   $913.10   $913.10