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 $6.73   6.449%   $7.164   $7.16   $7.15   $7.20  
  Purchase #2 $247.96   8.237%   $268.3845   $268.38   $268.40   $268.40  
  Purchase #3 $45.48   12.844%   $51.3215   $51.32   $51.30   $51.30  
  Purchase #4 $16.35   3.806%   $16.9723   $16.97   $16.95   $17.00  
  Purchase #5 $28.47   9.136%   $31.071   $31.07   $31.05   $31.10  
  Purchase #6 $88.39   3.197%   $91.2158   $91.22   $91.20   $91.20  
  Purchase #7 $136.65   13.472%   $155.0595   $155.06   $155.05   $155.10  
  7 purchase totals = $570.03       $621.1886   $621.18   $621.10   $621.30