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 $6.72   4.366%   $7.0134   $7.01   $7.00   $7.00  
  Purchase #2 $212.84   8.490%   $230.9101   $230.91   $230.90   $230.90  
  Purchase #3 $244.85   12.774%   $276.1271   $276.13   $276.15   $276.10  
  Purchase #4 $102.58   9.640%   $112.4687   $112.47   $112.45   $112.50  
  Purchase #5 $223.32   12.486%   $251.2037   $251.20   $251.20   $251.20  
  Purchase #6 $109.57   7.648%   $117.9499   $117.95   $117.95   $117.90  
  Purchase #7 $86.43   11.350%   $96.2398   $96.24   $96.25   $96.20  
  7 purchase totals = $986.31       $1,091.9127   $1,091.91   $1,091.90   $1,091.80