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 $227.54   6.111%   $241.445   $241.45   $241.45   $241.40  
  Purchase #2 $58.55   13.180%   $66.2669   $66.27   $66.25   $66.30  
  Purchase #3 $83.65   5.660%   $88.3846   $88.38   $88.40   $88.40  
  Purchase #4 $149.57   12.492%   $168.2543   $168.25   $168.25   $168.30  
  Purchase #5 $155.62   11.281%   $173.1755   $173.18   $173.20   $173.20  
  Purchase #6 $25.90   5.157%   $27.2357   $27.24   $27.25   $27.20  
  Purchase #7 $135.54   12.157%   $152.0176   $152.02   $152.00   $152.00  
  7 purchase totals = $836.37       $916.7796   $916.79   $916.80   $916.80