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 $84.27   4.803%   $88.3175   $88.32   $88.30   $88.30  
  Purchase #2 $235.82   6.825%   $251.9147   $251.91   $251.90   $251.90  
  Purchase #3 $106.55   3.732%   $110.5264   $110.53   $110.55   $110.50  
  Purchase #4 $36.72   11.773%   $41.043   $41.04   $41.05   $41.00  
  Purchase #5 $62.36   9.740%   $68.4339   $68.43   $68.45   $68.40  
  Purchase #6 $45.47   12.800%   $51.2902   $51.29   $51.30   $51.30  
  Purchase #7 $124.69   10.602%   $137.9096   $137.91   $137.90   $137.90  
  7 purchase totals = $695.88       $749.4353   $749.43   $749.45   $749.30