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 $79.75   13.277%   $90.3384   $90.34   $90.35   $90.30  
  Purchase #2 $145.52   12.109%   $163.141   $163.14   $163.15   $163.10  
  Purchase #3 $15.85   5.857%   $16.7783   $16.78   $16.80   $16.80  
  Purchase #4 $27.99   10.864%   $31.0308   $31.03   $31.05   $31.00  
  Purchase #5 $75.75   11.707%   $84.6181   $84.62   $84.60   $84.60  
  Purchase #6 $194.31   13.370%   $220.2892   $220.29   $220.30   $220.30  
  Purchase #7 $166.25   10.413%   $183.5616   $183.56   $183.55   $183.60  
  7 purchase totals = $705.42       $789.7574   $789.76   $789.80   $789.70