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 $11.72   4.173%   $12.2091   $12.21   $12.20   $12.20  
  Purchase #2 $36.30   7.813%   $39.1361   $39.14   $39.15   $39.10  
  Purchase #3 $57.92   5.436%   $61.0685   $61.07   $61.05   $61.10  
  Purchase #4 $191.62   11.381%   $213.4283   $213.43   $213.45   $213.40  
  Purchase #5 $72.79   10.442%   $80.3907   $80.39   $80.40   $80.40  
  Purchase #6 $22.91   10.442%   $25.3023   $25.30   $25.30   $25.30  
  Purchase #7 $90.70   9.792%   $99.5813   $99.58   $99.60   $99.60  
  7 purchase totals = $483.96       $531.1163   $531.12   $531.15   $531.10